Alloy material for thixocasting, process for preparing semi-molten alloy material for thixocasting and thixocasting process

ABSTRACT

A differential thermal analysis thermograph for a thixocasting aluminum alloy material is such that, a relationship, E 1  &gt;E 2 , is established between a peak value E 1  of a first angled endothermic section generated by eutectic melting and a peak value E 2  of a second angled endothermic section generated by melting of an element having a melting point higher than the eutectic point. Thus, a liquid phase produced by the eutectic melting has a large latent heat due to the fact that the peak value E 1  of the first angled endothermic section is larger than the peak value E 2  of the second angled endothermic section in a semi-molten eutectic aluminum alloy material derived from the thixocasting aluminum alloy material. As a result, in a solidification step of the thixocasting process, a liquid phase produced by the eutectic melting is sufficiently fed around a solid phase in response to the solidification and shrinkage of the solid phase, and then solidified. Thus, it is possible to produce a casting free from defects such as voids on the order of a micron.

This is a continuation of application Ser. No. 08/441,158 filed May 15, 1995, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an alloy material for thixocasting, a process for preparing a semi-molten alloy material for thixocasting, and a thixocasting process.

2. Description of the Prior Art

Conventionally known alloy materials for thixocasting such as aluminum alloys include AA specification 2000-series alloys used to provide an enhancement in heat resistance of a casting, and AA specification 6000-series alloys used to provide increases in strength and toughness of a casting.

In carrying out the thixocasting process, a procedure is employed which involves subjecting an aluminum alloy material to a heating treatment to prepare a semi-molten aluminum alloy material having a solid phase (which hereinafter indicates a substantially solid phase) and a liquid phase coexisting therein, pouring the semi-molten aluminum alloy material under pressure into a cavity in a casting mold, and solidifying the semi-molten aluminum alloy material under the pressure.

In carrying out the thixocasting process, however, the following problem is encountered: when a conventional aluminum alloy material is used, defects such as voids on the order of a micron are liable to be generated at the boundary between the granular solid phases of the casting.

The generation of such defects is particularly prominent at thick portions of the casting and where casting has complicated shapes and the like, and causes a reduction in the fatigue strength of the casting.

The known materials of this type include alloy materials, that have, in a differential thermal analysis thermograph both a first angled endothermic section generated by eutectic melting and a second angled endothermic section generated by the melting of a segment having a melting point higher than the eutectic point, and a difference T₅ -T₄ between a peak temperature T₄ of the first angled endothermic section and a peak temperature T₅ of the second angled endothermic section in a range represented by T₅ -T₄ <20° C.

The reason why the difference T₅ -T₄ between the temperatures T₄ and T₅ is set at such a small value as described above is that the range of solid-liquid coexistence temperature of the alloy material is narrowed to inhibit the generation of a shrinkage cavity during solidification.

However, in the alloy material having a temperature difference T₅ -T₄ set at such a small value, the solid phase proportion is varied in accordance with a variation in heating temperature. For this reason, in order to uniformly heat such an alloy material, the temperature rising rate must be reduced, namely, the heating time must be set at a large value. As a result, there is a disadvantage of including a coalescence of primary crystals which are main segments for the solid phase. On the other hand, if the temperature rising rate is increased, i.e., if the heating time is set short, the distribution of solid proportion in the semi-molten alloy material is not uniform, resulting in a disadvantage that the semi-molten alloy material is charged in a sprayed state into the cavity in the casting course and as a result, air is included into the semi-molten alloy material to generate relatively large voids.

In this type of alloy material, the range of temperature permitting the liquid phase due to the eutectic melting to exist in the course of solidification, is set at a relatively narrow range in order to avoid the generation of shrinkage cavities.

In the heating treatment of the alloy material, it is necessary to increase the temperature rising rate for the alloy material to rapidly heat the alloy material within a short time up to a casting temperature in order to provide an increase in efficiency.

However, if the conventional alloy material is rapidly heated, the following problem is encountered: it is impossible to flow the liquid phase to uniformize the distribution of the liquid phase in the course of solidification, because the range of temperature permitting the liquid phase to exist is relatively narrow. As a result, the metallographic structure of the produced casting is not uniform and hence, the casting has a low fatigue strength.

There is a conventionally known process for preparing a semi-molten alloy material for thixocasting by heating an alloy material having a first angled endothermic section generated by eutectic melting and a second angled endothermic section generated by melting of an element having a melting point higher than the eutectic point in a differential thermal analysis thermograph, to a casting temperature between a dropping end point in the first angled endothermic section and a peak of the second angled endothermic section, so that solid and liquid phases coexist in the semi-molten alloy material. In this case, the average temperature rising rate for the alloy material is set to be substantially constant and considerably high. The reason why the average temperature rising rate is set in this manner is that the coalescence of the metallographic structure due to the growth of a primary crystal or the like is prevented to enhance the heating performance.

However, the known process suffers from a problem that the profile of the temperature of the liquid phase in the semi-molten alloy material is liable to become non-uniform, and due to this, voids on the order of a micron are generated and for this reason, it is difficult to produce a casting having a high grade of mechanical properties such as an excellent fatigue strength.

In carrying out the thixocasting process, it is a conventional practice to set the temperature of a semi-molten alloy material during casting, i.e., the casting temperature at a level between a rising start point in the first angled endothermic section and a dropping end point in the first angled endothermic section to maintain the semi-molten alloy material at a high solid phase proportion in order to improve the handling ability of the semi-molten alloy material.

However, if the thixocasting process is carried out under a high solid phase proportion as in the known process, the following problem is encountered: despite the use of, for example, a hypoeutectic alloy material, a primary crystal of alloy elements is precipitated with an eutectic reaction of the semi-molten alloy material, and due to this, a casting having a reduced ductility and toughness is produced. This phenomenon is generated in an Al-Si alloy, an Al-CuAl₂ alloy, an Al-Mg₂ Si alloy and the like.

SUMMARY OF THE INVENTION

The present inventors have made studies to solve the above problems and as a result, they have found that the defects of the casting is due to the liquid phase being insufficiently fed around the solid phase in response to the solidification and shrinkage of the solid phase, because of the small latent heat of the liquid phase in the semi-molten aluminum alloy material.

The present invention has been accomplished with the result of the studies taken into account, and it is an object of the present invention to provide an alloy material of the above-described type, from which a casting free from defects and having a sound quality can be produced in a thixocasting process.

To achieve the above object, according to the present invention, there is provided an alloy material for thixocasting, wherein in a differential thermal analysis thermograph, a peak value E₁ of a first angled endothermic section generated by eutectic melting is larger than a peak value E₂ of a second angled endothermic section generated by melting of an element having a melting point higher than the eutectic point.

A semi-molten alloy material having solid and liquid phases coexisting therein is prepared by subjecting the alloy material to a heating treatment. In this semi-molten alloy material, the liquid phase produced by the eutectic melting has a large latent heat due to the fact that the peak value E₁ of the first angled endothermic section is larger than the peak value E₂ of the second angled endothermic section. As a result, the liquid phase is sufficiently fed around the solid phase and then solidified in response to the solidification and shrinkage of the solid phase in the course of solidification of the thixocasting process. Thus, it is possible to produce a casting free from defects, such as voids on the order of a micron, and having a sound casting quality.

The present inventors have made studies to solve the above-described problems and consequently have found that the defects of the casting is also due to a degraded compatibility between the solid and liquid phases because the latent heat of the liquid phase in the semi-molten alloy material is small and hence, the liquid phase is insufficiently fed around the solid phase in response to the solidification and shrinkage of the solid phase. Furthermore, an outer periphery of the solid phase is substantially in the solid state.

The present invention has been accomplished with the result of the studies taken into account, and it is an object of the present invention to provide an alloy material of the above-described type, which exhibits a good feedability of a liquid phase to a solid phase and a good compatibility between solid and liquid phases in a thixocasting process, and from which a casting free from defects and having a sound casting quality and a high fatigue strength can be produced in the thixocasting process.

To achieve the above object, according to the present invention, there is provided an alloy material for thixocasting, wherein in a differential thermal analysis thermograph, a ratio E₁ /E₂ of a peak value E₁ of a first angled endothermic section generated by eutectic melting to a peak value E₂ of a second angled endothermic section generated by melting of an element having a melting point higher than an eutectic point is in a range of 1<E₁ /E₂ <2.5, and a difference T₂ -T₁ between a temperature T₁ at a rising start point in the first angled endothermic section and a temperature T₂ at a dropping end point in the second angled endothermic section is in a range of 10° C.<T₂ -T₁ <120° C.

A semi-molten alloy material including liquid and solid phases coexisting and having a eutectic composition is prepared by subjecting the above described alloy material to a heating treatment. In this semi-molten alloy material, the liquid phase has a large latent heat due to the fact that the ratio E₁ /E₂ of the peak values E₁ and E₂ is as specified above. As a result, the liquid phase is sufficiently fed around the solid phase and then solidified in response to the solidification and shrinkage of the solid phase in the course of solidification of the thixocasting process. The outer periphery of the solid phase is in a gelled state due to the fact that the difference T₂ -T₁ between the temperatures is specified. As a result, the compatibility between the gelled outer periphery of the solid phase and the liquid phase is improved. Thus, it is possible to prevent the generation of voids on the order of a micron in the casting.

The quantity balance between the solid and liquid phases is excellent due to the ratio E₁ /E₂ specified in the above manner and hence, the semi-molten alloy material becomes a homogeneous fluid in the course of casting. Thus, it is possible to prevent the generation of a segregation in the casting. Such a casting has a sound casting quality and a high fatigue strength.

However, if the ratio E₁ /E₂ of the peak value E₁ to the peak value E₂ is equal to or smaller than 1 (E₁ /E₂ ≦1), the latent heat of the liquid phase is decreased and for this reason, the feeding of the liquid phase around the solid phase is insufficient in the course of the solidification and shrinkage. As a result, voids on the order of a micron are liable to be produced in the casting. On the other hand, if E₁ /E₂ ≧2.5, a primary crystal is segregated and coalesced and hence, voids are likewise liable to be generated. The segregation of the primary crystal is due to the fact that the semi-molten alloy material becomes an inhomogeneous fluid in the course of casting (e.g., during passing of the alloy material through a gate of a mold), because of a large amount of the liquid phase. On the other hand, the coalescence of the primary crystal is due to the fact that the cooling of the primary crystal is delayed, when an excessive amount of the liquid phase is solidified, because of a large latent heat of the liquid phase.

If the difference T₂ -T₁ between the temperatures is equal to or lower than 10° C., a solid-liquid coexisting state having a predetermined solid proportion cannot be maintained stably, and the gelling of the outer periphery of the solid phase is sufficient. As a result, voids are likewise liable to be produced. On the other hand, if T₂ -T₁ ≧120° C., shrinkage cavities are liable to be generated in the casting, because of a prolonged time of course of solidification of the liquid phase in the solidification.

It is a further object of the present invention to provide an alloy material of the above-described type, which exhibits a good feedability of a liquid phase to a solid phase, and from which a casting free from defects and having a sound casting quality and a high fatigue strength can be produced in the thixocasting process.

To achieve the above object, according to the present invention, there is provided an alloy material for thixocasting, wherein in a differential thermal analysis thermograph, a relationship of E₁ >E₂ is established between a peak value E₁ of a first angled endothermic section generated by an eutectic melting and a peak value E₂ of a second angled endothermic section generated by melting of an element having a melting point higher than an eutectic point. Further, when a temperature straight line interconnecting a peak of the first angled endothermic section and a temperature graduation of the peak on an axis of heating temperature intersects a basic line which interconnects a rising start point in the first angled endothermic section and a dropping end point in the second angled endothermic section, and when a dividing line bisecting one segment of the temperature straight line lying between the peak and a first intersection of the temperature straight line with the basic line intersects a rising line segment of the first angled endothermic section lying between the rising start point and the peak, a relationship of .increment.Tb/.increment.Ta≦0.68 is established between .increment.Ta (=T₄ -T₁) and .increment.Tb (=T₃ -T₁), wherein T₁ represents a temperature at the rising start point, T₃ represents a temperature at a second intersection between the rising line segment and the dividing line, and T₄ represents the peak temperature.

According to the present invention, there is also provided an alloy material for thixocasting, wherein a relationship of E₁ >E₂ is established between a peak value E₁ of a first angled endothermic section generated by eutectic melting and a peak value E₂ of a second angled endothermic section generated by melting of a segment having a melting point higher than the eutectic point in a differential thermal analysis thermograph. Further, a relationship of Sm/St≦0.365 is established between (1) an area St of a region surrounded by (i) a basic line which interconnects a rising start point of the first angled endothermic section and a dropping end point in the second angled endothermic section, (ii) a first temperature straight line which interconnects a peak of the first angled endothermic section and a temperature graduation of the peak on an axis of heating temperature, and (iii) a rising line segment of the first angled endothermic section lying between the rising start point and said peak, and (2) an area Sm of a region surrounded by (i) a second temperature straight line which interconnects (a) a second intersection of the rising line segment with a dividing line which bisects a segment of the first temperature straight line lying between a first intersection of the first temperature straight line with the basic line and the peak, and (b) a temperature graduation of the second intersection on the axis of heating temperature, (ii) a portion of the rising line segment lying between the rising start point of said first angled endothermic section and the second intersection, and (iii) the basic line.

A semi-molten alloy material having solid and liquid phases coexisting therein is prepared by subjecting the alloy material to a heating treatment. In this semi-molten alloy material, the liquid phase produced by eutectic melting has a large latent heat due to the fact that the peak value E₁ of the first angled endothermic section is larger than the peak value E₂ of the second endothermic section. As a result, in the course of solidification in the thixocasting process, the liquid phase is sufficiently fed around the solid phase and then solidified. Thus, it is possible to prevent the generation of voids on the order of a micron in a relatively thin portion, a portion of a simple shape or the like of a casting.

If the latent heat of the liquid phase is reduced to 1/2 of the maximum released latent heat, namely to E₁ /2 in the course of solidification of the semi-molten alloy material, the liquid phase is thereafter gelled, so that the viscosity is gradually increased. Due to this, the feedability of the liquid phase around the solid phase, particularly in a thicker portion, a portion of a complicated shape or the like of a casting is degraded and hence, voids on the order of a micron are liable to be generated in the thicker portion or the like.

The value .increment.Ta represents a temperature drop range required for the liquid phase to be solidified, and the value .increment.Tb represents a temperature drop range required for the gelled phase produced from the liquid phase to be solidified. Therefore, if .increment.Tb/.increment.Ta is set in a range of .increment.Tb/.increment.Ta≦0.68, the temperature range required for the gelled phase to be solidified is narrowed, and on the other hand, the temperature range permitting the gelled phase to be produced from the liquid phase is relatively widened. Thus, it is possible to improve the feedability of the liquid phase around the solid phase in the thicker portion of the casting or the like to prevent the generation of voids on the order of a micron.

The area St indicates a released latent heat amount required for the liquid phase to be solidified, and the area Sm indicates a released latent heat amount required for the gelled phase produced from the liquid phase to be solidified. Therefore, if Sm/St is set in a range of Sm/St ≦0.365, the released latent heat amount required for the gelled phase to be solidified is decreased, and on the other hand, the released latent heat amount permitting the gelled phase to be produced from the liquid phase is relatively increased. Thus, it is possible to improve the feedability of the liquid phase around the solid phase in the thicker portion of the casting or the like to prevent the generation of voids on the order of a micron. Such a casting has a sound casting quality.

Further, it is another object of the present invention to provide an alloy material for thixocasting of the above-described type, from which a casting having a sound casting quality and higher mechanical properties such as an excellent fatigue strength can be produced by specifying the difference T₅ -T₄ between the peak temperatures T₄ and T₅ of the first and second angle endothermic sections.

To achieve the above object, according to the present invention, there is provided an alloy material for thixocasting, wherein in a differential thermal analysis thermograph, there exist a first angled endothermic section generated by eutectic melting and a second angled endothermic section generated by melting of a segment having a melting point higher than the eutectic point, and wherein a difference T₅ -T₄ between a peak temperature T₄ of the first angled endothermic section and a peak temperature T₅ of the second angled endothermic section is in a range of 20° C.≦T₅ -T₄ ≦80° C.

In the alloy material having the characteristic that the difference T₅ -T₄ between the temperatures T₄ and T₅ is specified in the above range, the variation in solid phase proportion relative to the variation in heating temperature is blunted. Therefore, it is possible to increase the temperature rising rate to heat the alloy material up to a casting temperature (which hereinafter indicates a temperature of the material during casting) within a short time, thereby preventing the coalescence of the primary crystal.

In addition, the diffusion of the primary crystal is actively effected as a result of widening of the temperature range permitting the solid and liquid phases to coexist, and therefore, the fine spheroidization and the uniform dispersion of the primary crystal are promoted. This brings about the uniformization of the metallographic structure in the semi-molten alloy material, so that the uniform solidification of the material is achieved, thereby avoiding the generation of shrinkage cavities in a casting. Such a casting has a sound casting quality and high mechanical properties such as an excellent fatigue strength.

However, if the difference T₅ -T₄ between the temperatures is lower than 20° C., the above-described disadvantage arises. On the other hand, if T₅ -T₄ >80° C., the solid/liquid phase coexisting temperature range of the alloy material is too wide, and hence, shrinkage cavities are liable to be generated during solidification.

It is a further object of the present invention to provide an alloy material of the above-described type, from which a casting having a uniform metallographic structure can be produced, even when the alloy material is rapidly heated.

To achieve the above object, according to the present invention, there is provided an alloy material for thixocasting, wherein in a differential thermal analysis thermograph, there exist a first angled endothermic section generated by eutectic melting and a second angled endothermic section generated by melting of a segment having a melting point higher than the eutectic point, the second angled endothermic section having a sloping portion between a peak of the second angled endothermic section and a dropping end point in the second angled endothermic section, the sloping portion extending along a straight line interconnecting a rising start point in the first angled endothermic section and the dropping end point in the second angled endothermic section. Further, a proportion Rt (Td/Tc×100) of a temperature range Td between the peaks of the first and second angled endothermic sections and in a solid-liquid phase-coexisting temperature range Tc between the rising start point in the first angled endothermic section and the dropping end point in the second angled endothermic section is in a range of RT≧69%.

If the second angled endothermic section has the gentle-sloping portion in the differential thermal analysis thermograph, and the proportion Rt (Td/Tc×100) of a peak-peak temperature range Td between the peaks of the first and second angled endothermic sections in the solid-liquid phase coexisting temperature range Tc is set in the above-described range, the temperature range permitting a liquid phase due to the eutectic melting to exist in the course of solidification is widened. Therefore, even when the alloy material is rapidly heated, it is possible to sufficiently flow the liquid phase between the solid phases, thereby uniformizing the distribution of the liquid phase.

Thus, even when the uniformization of heat in the semi-molten alloy material is not achieved due to the rapid heating, the fluidity of the material can be improved to enhance the moldability, and a casting having a uniform metallographic structure and a high fatigue strength can be produced. In this case, because the distribution of the liquid phase is uniformized, the solidification of the semi-molten alloy material is effected substantially over the whole thereof and therefore, the generation of shrinkage cavities is avoided.

However, if a gentle-sloping portion as described above does not exist in the differential thermal analysis thermograph, and Rt<69%, a casting produced through the rapid heating has a non-uniform metallographic structure.

Further, it is an object of the present invention to provide a producing process of the above-described type, wherein in subjecting the alloy material to a heating treatment, the average temperature rising rate can be controlled in correspondence to the thermal characteristic of the alloy material, thereby enabling the preparation of a semi-molten alloy material from which a casting having a sound casting quality can be obtained.

To achieve the above object, according to the present invention, there is provided a process for preparing a semi-molten alloy material for thixocasting having solid and liquid phases coexisting therein, comprising the step of heating an alloy material having such a characteristic that a first angled endothermic section generated by eutectic melting and a second endothermic section generated by melting of a segment having a melting point higher than the eutectic point exist in a differential thermal analysis thermograph, to a casting temperature laying between a dropping end point in the first angled endothermic section and a peak of the second endothermic section, wherein a ratio α/β between the following two average temperature rising rates α and β is set in a range of α/β1: (1) the average temperature rising rate α for the alloy material between a rising start point and the dropping end point in the first angled endothermic section, and (2) the average temperature rising rate β for the alloy material between the dropping end point and the casting temperature.

If the ratio α/β between the average temperature rising rates α and β is set in the range of α/β>1, the eutectic melting is performed at the relatively high average temperature rising rate α and therefore, the coalescence of the metallographic structure due to the growth and/or the coalescence of a primary crystal is suppressed. On the other hand, after the eutectic melting, the heating treatment advances at the relatively low average temperature rising rate β and therefore, the formation of a liquid phase of an eutectic segment is promoted, and the profile of temperature of the liquid phase is uniformized. Thus, it is possible to produce a casting having a sound casting quality.

However, if α/β≦1, the metallographic structure is liable to be coalesced due to the growth of the primary crystal or the like, and the profile of temperature of the liquid phase is liable to become non-uniform.

Further, the present inventors have made studies in order to solve the above-described problems in the thixocasting process, and as a result, they have found that the precipitation of a primary crystal of the alloy elements is caused by a high cooling rate during an eutectic reaction of the semi-molten alloy material.

The present invention has been accomplished with the result of the studies taken into account, and it is an object of the present invention to provide a thixocasting process of the above-described type, wherein the cooling rate during the eutectic reaction of the semi-molten alloy material can be reduced to avoid the precipitation of the primary crystal of the alloy elements, thereby producing a casting having a high ductility and a high toughness.

To achieve the above object, according to the present invention, there is provided a thixocasting process, using an alloy material in which in a differential thermal analysis thermograph, there exist a first angled endothermic section generated by eutectic melting and a second angled endothermic section generated by melting of a segment having a melting point higher than the eutectic point, wherein a casting temperature T₆ of the alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping end point in the first angled endothermic section, and T₅ represents a peak temperature of the second angled endothermic section.

If the casting temperature T₆ of the alloy material is set in the above range, the alloy material is brought into a semi-molten state and moreover, the semi-molten alloy material has a lowered solid phase proportion. Namely, the liquid phase amount is relatively increased.

As a result, the cooling rate is lowered in the eutectic reaction of the semi-molten alloy material, because the relatively large amount of the liquid phase has a relatively large latent heat. This makes it possible to avoid the precipitation of a primary crystal of alloy elements.

A casting produced in this manner has a high ductility and a high toughness.

However, if the casting temperature T₆ is lower than T₇, the precipitation of the primary crystal of the alloy elements to occurs. On the other hand, if T₆ ->T₅, the shape retention of the semi-molten alloy material is reduced, resulting in a degraded handling ability thereof.

The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a pressure casting apparatus;

FIG. 2 is a phase diagram of an Al-Cu based alloy;

FIG. 3 is a differential thermal analysis thermograph for an example A;

FIG. 4A illustrates an essential portion of the differential thermal analysis thermograph for the example A;

FIG. 4B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A;

FIG. 5A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₁ ;

FIG. 5B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₁ ;

FIG. 6A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₂ ;

FIG. 6B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₂ ;

FIG. 7 is a diagram illustrating a semi-molten state of an example;

FIG. 8 is a phase diagram of an Al-Si based alloy;

FIG. 9A illustrates an essential portion of the differential thermal analysis thermograph for the example A₁ ;

FIG. 9B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁ ;

FIG. 10A illustrates an essential portion of the differential thermal analysis thermograph for the example A₂ ;

FIG. 10B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₂ ;

FIG. 11A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a;

FIG. 11B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a;

FIG. 12 is a differential thermal analysis thermograph for an example A₁ ;

FIG. 13 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁ ;

FIG. 14A illustrates an essential portion of a differential thermal analysis thermograph for an example A₂ ;

FIG. 14B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₂ ;

FIG. 15A illustrates an essential portion of a differential thermal analysis thermograph for an example A₃ ;

FIG. 15B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₃ ;

FIG. 16A illustrates an essential portion of a differential thermal analysis thermograph for an example A₄ ;

FIG. 16B is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₄ ;

FIG. 17A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₁ ;

FIG. 17B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₁ ;

FIG. 18A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₂ ;

FIG. 18B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₂ ;

FIG. 19A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₃ ;

FIG. 19B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₃ ;

FIG. 19C schematically illustrates a portion of the photomicrograph shown in FIG. 19B.

FIG. 20A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₄ ;

FIG. 20B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₄ ;

FIG. 21A illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₅ ;

FIG. 21B is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₅ ;

FIG. 22 is a graph illustrating fatigue strengths of various aluminum alloy castings;

FIG. 23 is a sectional view of an aluminum alloy casting;

FIG. 24 is a vertical sectional view of a pressure casting device;

FIG. 25 is a differential thermal analysis thermograph for an example A₁ ;

FIG. 26 is an enlarged diagram of an essential portion shown in FIG. 25;

FIG. 27 is a differential thermal analysis thermograph for an example A₂ ;

FIG. 28 is a differential thermal analysis thermograph for an example A₃ ;

FIG. 29 is an enlarged diagram of an essential portion shown in FIG. 28;

FIG. 30 is a differential thermal analysis thermograph for a comparative example a₁ ;

FIG. 31 is a differential thermal analysis thermograph for a comparative example a₂ ;

FIG. 32 is a photomicrograph showing the metallographic structure of an aluminum alloy casting produced using the example A₁ ;

FIG. 33 is a photomicrograph showing the metallographic structure of an aluminum alloy casting produced using the example A₂ ;

FIG. 34A is a photomicrograph showing the metallographic structure of an aluminum alloy casting produced using the comparative example a₁ ;

FIG. 34B is an essential portion of the photomicrograph shown in FIG. 34A;

FIG. 35 is a graph illustrating the relationship between the ratio .increment.Tb/.increment.Ta and the area proportion of defects in a thicker portion;

FIG. 36 is a graph illustrating the relationship between the ratio Sm/St and the area proportion of defects in a thicker portion;

FIG. 37 illustrates an essential portion of a differential thermal analysis thermograph for an example A_(l) ;

FIG. 38 illustrates an essential portion of a differential thermal analysis thermograph for an example A₂ ;

FIG. 39 illustrates an essential portion of a differential thermal analysis thermograph for an example A₃ ;

FIG. 40 illustrates an essential portion of a differential thermal analysis thermograph for an example A₄ ;

FIG. 41 illustrates an essential portion of a differential thermal analysis thermograph for an example A₅ ;

FIG. 42 illustrates an essential portion of a differential thermal analysis thermograph for an example A₆ ;

FIG. 43 illustrates an essential portion of a differential thermal analysis thermograph for an example A₇ ;

FIG. 44 illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₁ ;

FIG. 45 illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₂ ;

FIG. 46 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₇ ;

FIG. 47A is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₁ ;

FIG. 47B illustrates an essential portion of the photomicrograph shown in FIG. 47A;

FIG. 48 is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₃ ;

FIG. 49 is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₂ ;

FIG. 50 is a graph illustrating the relationship between the temperature difference T₅ -T₄ and the strength ratio S₁ /S₂ ;

FIG. 51 illustrates an essential portion of a differential thermal analysis thermograph for an example A₁ ;

FIG. 52 illustrates an essential portion of a differential thermal analysis thermograph for an example A₂ ;

FIG. 53 illustrates an essential portion of a differential thermal analysis thermograph for an example A₃ ;

FIG. 54 illustrates an essential portion of a differential thermal analysis thermograph for an example A₄ ;

FIG. 55 illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₁ ;

FIG. 56 illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₂ ;

FIG. 57 illustrates an essential portion of a differential thermal analysis thermograph for a comparative example a₃ ;

FIG. 58 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁ ;

FIG. 59 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₂ ;

FIG. 60 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₄ ;

FIG. 61 is a photomicrograph showing the metallographic structure of an aluminum alloy casting a₁ ;

FIG. 62 is a graph illustrating the relationship between the proportion Rt of a peak-peak temperature range Td in a solid-liquid phase coexisting temperature range Tc and the fatigue strength ratio;

FIG. 63 illustrates an essential portion of a differential thermal analysis thermograph for an example A₁ ;

FIG. 64 is a graph illustrating the relationship between the heating time and the temperature of the example A₁ of an aluminum alloy material;

FIG. 65 illustrates an essential portion of a differential thermal analysis thermograph for an example A₂ ;

FIG. 66 illustrates an essential portion of a differential thermal analysis thermograph for an example A₃ ;

FIG. 67 illustrates an essential portion of a differential thermal analysis thermograph for an example A₄ ;

FIG. 68 illustrates an essential portion of a differential thermal analysis thermograph for an example A₅ ;

FIG. 69A is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁₁ ;

FIG. 69B is a diagram for explaining how to measure the size of α-Al;

FIG. 70A is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁₂ ;

FIG. 70B is a diagram for explaining how to measure the size of α-Al;

FIG. 71 is a graph illustrating fatigue strengths of various aluminum alloy castings;

FIG. 72 is a differential thermal analysis thermograph for an aluminum alloy material;

FIG. 73 illustrates an essential portion of a cooling curve for the aluminum alloy material;

FIG. 74 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₁ ;

FIG. 75 is a photomicrograph showing the metallographic structure of an aluminum alloy casting A₅ ;

FIG. 76 is a graph illustrating the relationship between the casting temperature T₆, the cooling rate during an eutectic reaction, the breaking extension and the Charpy impact value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a pressure casting apparatus 1 used for producing an aluminum alloy casting in a thixocasting process using an aluminum alloy material. The pressure casting apparatus 1 includes a stationary die 2 and a movable die 3 having vertical matched faces 2a and 3a. A casting forming cavity 4 is defined between the matched faces 2a and 3a. A chamber 6 for placement of an aluminum alloy material 5 is defined in the stationary die 2 and communicates with a lower portion of the cavity 4 through a gate 7. A sleeve 8 is horizontally mounted on the stationary die 2 to communicate with the chamber 6, and a pressurizing plunger 9 is slidably received in the sleeve 8 and insertable into and removable from the chamber 6. The sleeve 8 has a material charging port 10 in an upper portion of a peripheral wall thereof.

Embodiment I Example 1

In the example 1, a hypoeutectic Al-Cu based alloy material will be mainly described.

(A) FIG. 2 shows a phase diagram of an Al-Cu based alloy, and Table 1 shows compositions of an example A and comparative examples a₁ and a₂ of Al-Cu based alloy materials. In FIG. 2, A, a₁ and a₂ correspond to the example A and the comparative examples a₁ and a₂, respectively. Each of the example A and the comparative examples a₁ and a₂ was cut off from a high quality continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the example A and the comparative examples a₁ and a₂ has a diameter of 76 mm and a length of 85 mm.

                                      TABLE 1                                      __________________________________________________________________________     Chemical constituents (% by weight)                                            Si         Cu  Mg  Fe  Zr Zn Mn V  Ni  Ti   balance                            __________________________________________________________________________     Example A                                                                            0.17 10.3                                                                               --  0.18                                                                               0.18                                                                              -- 0.27                                                                              0.1                                                                               --  0.05 Al                                 Comparative                                                                          0.17 6.3 --  0.19                                                                               0.08                                                                              -- 0.28                                                                              0.11                                                                              --  0.05 Al                                 example a.sub.1                                                                Comparative                                                                          0.1˜0.25                                                                      1.9˜2.7                                                                      1.3˜1.8                                                                      0.9˜1.3                                                                      -- <0.1                                                                              -- -- 0.9˜1.2                                                                      0.04˜0.1                                                                      Al                                 example a.sub.2                                                                (2618 alloy)                                                                   __________________________________________________________________________

The example A was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 3. In the differential thermal analysis thermograph of FIG. 3, a peak value E₁ of a first angled endothermic section e due to an eutectic melting is equal to 2.7 mcal/sec, and a peak value E₂ of a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point is equal to 2.6 mcal/sec. It was confirmed from the above that E₁ >E₂. Likewise, the comparative examples a₁ and a₂ were subjected to the differential scanning calorimetry (DSC) to provide results which will be described hereinafter.

Then, the example A was placed into a heating coil in an induction heating device and then heated under conditions of a frequency of 1 kHz and a maximum output power of 37 kW to prepare a semi-molten example A having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 50% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A (indicated by character 5 in FIG. 1) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a moving velocity of the pressurizing plunger 9 of 0.07 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A was solidified under the pressurization to provide an aluminum alloy casting A. Aluminum alloy castings a₁ and a₂ were also produced in a similar casting manner using the comparative examples a₁ and a₂. FIGS. 4A and 4b to 6A and 6B show essential portions of differential thermal analysis thermographs for the example A and the comparative examples a₁ and a₂ and photomicrographs of the metallographic structures of the aluminum alloy castings A, a₁ and a₂.

In the case of the example A, a relationship, E₁ >E₂ as described above was established between the peak values E₁ and E₂, as shown in FIG. 4A, and from this fact, no defect such as voids on the order of a micron was produced in the aluminum alloy casting A, as shown in FIG. 4B.

In the case of the comparative example a₁, the peak value E₁ of the first angle endothermic section e is equal to 0.61 mcal/sec, and the peak value E₂ of the second angle endothermic section f is equal to 5.6 mcal/sec, as shown in FIG. 5A, and hence, E₁ <E₂. Due to this, the latent heat of the liquid phase is lower than that of the example A. As a result, a large number of voids on the order of a micron were generated at boundaries between the granular solid phases in the aluminum alloy casting a₁, as shown in FIG. 5B.

In the case of the comparative example a₂, an eutectic melting did not occur (see FIG. 2), as shown in FIG. 6A and hence, only one angled endothermic section was generated. Due to this, the latent heat of the liquid phase is low, as compared with the comparative example a₁ and as a result, relatively large voids were generated at boundaries between the granular solid phases in the aluminum alloy casting a₂, as shown in FIG. 6B.

(B) In a differential thermal analysis thermograph d shown in FIG. 3, a dropping line segment s of the first angled endothermic section e and a rising line segment w of the second angled endothermic section f are connected to each other in a region Ed of an endotherm higher than a basic line i which interconnects a rising start point g in the first angled endothermic section e and a dropping end point h in the second angled endothermic section f.

In the case of the example A such that a differential thermal analysis thermograph d is as described above, an outer periphery 12 of a solid phase 11 is gelled in the semi-molten state of the example A, as shown in FIG. 7, so that the compatibility between the gelled outer periphery 12 and a liquid phase 13 is improved. As a result, the strength of bonding between the granular solid phase which is a solidified phase derived from the solid phase 11 and a matrix which is a solidified phase derived from the solid phase 13, is increased in the aluminum alloy casting A, and thus, an increase in strength of the aluminum alloy casting A is achieved.

In the comparative example a₁, the dropping line segment s of the first angled endothermic section e and the rising line segment w of the second angled endothermic section f are connected to each other on the basic line i, as shown in FIG. 5A. In this case, the outer periphery of the solid phase is little gelled.

Then, using the example A and the comparative example a₁, five aluminum alloy castings A and a₁ were produced by a casting process similar to that described above and then subjected to a tensile test to provide results given in Table

                  TABLE 2                                                          ______________________________________                                                       Tensile strength (MPa)                                           ______________________________________                                         Al alloy casting A                                                             No. 1           307                                                            No. 2           357                                                            No. 3           359                                                            No. 4           337                                                            No. 5           343                                                            Average tensile strength                                                                       341                                                            Al alloy casting a.sub.1                                                       No. 1           289                                                            No. 2           251                                                            No. 3           301                                                            No. 4           274                                                            No. 5           212                                                            Average tensile strength                                                                       265                                                            ______________________________________                                    

It can be seen from Table 2 that the aluminum alloy casting A produced using the example A has a high strength, as compared with the aluminum alloy casting a₁ produced using the comparative example a₁. In place of the gelling of the outer periphery 12 of the solid phase 11, a segment, which can be deposited on the solid phase and gelled in the above-described situation, may be contained in a material.

(C) In the differential thermal analysis thermograph d as shown in FIG. 3, the gradient of the rising line segment of the second angled endothermic section f is gentler than the second angled endothermic section f is gentler than that of the dropping line segment s of the first angled endothermic section e.

In the case of the example A such that a differential thermal analysis thermograph d is as described above, the acceptable range of the casting temperature is widened, because the range of the casting temperature is set from a dropping end point u of the first angled endothermic section e to a peak v of the second angled endothermic section f. Thus, it is possible to stabilize the casting quality of an aluminum alloy casting.

Example 2

In this example 2, a hypoeutectic Al-Si based alloy material will be mainly described.

FIG. 8 shows a phase diagram of an Al-Si base alloy, and Table 3 shows compositions of examples A₁ and A₂ and a comparative example a of Al-Si based alloy materials. In FIG. 8, reference characters A₁, A₂ and a correspond to the examples A₁ and A₂ and the comparative example a, respectively. Each of the examples A₁ and A₂ and the comparative example a was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the examples A₁ and A₂ and the comparative example a has a diameter of 76 mm and a length of 85 mm.

                                      TABLE 3                                      __________________________________________________________________________     Chemical constituents (% by weight)                                            Si        Cu   Mg  Fe Zr Zn Mn V Ni                                                                               Ti   balance                                __________________________________________________________________________     Example A.sub.1                                                                      2.58                                                                               2.0  4.0 0.13                                                                              0.02                                                                              -- 0.01                                                                              --                                                                               --                                                                               0.03 Al                                     Example A.sub.2                                                                      6.5˜7.5                                                                      <0.2 0.4˜0.7                                                                      <0.2                                                                              -- -- -- --                                                                               --                                                                               0.04˜0.20                                                                     Al                                     Comparative                                                                          0.4˜0.8                                                                      0.15˜0.4                                                                      0.8˜1.2                                                                      <0.7                                                                              -- <0.25                                                                             <0.15                                                                             --                                                                               --                                                                               <0.15                                                                               Al                                     example a                                                                      (6061 alloy)                                                                   __________________________________________________________________________

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 9A. In the differential thermal analysis thermograph of FIG. 9A, a peak value E₁ of a first angled endothermic section e due to an eutectic melting is equal to 3.2 mcal/sec, and a peak value E₂ of a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point is equal to 2.9 mcal/sec. It was confirmed from the above that E₁ >E₂. Likewise, the example A₂ and the comparative example a were subjected to a similar differential scanning calorimetry (DSC) to provide results which will be described hereinafter.

Then, the example A₁ was placed into the heating coil in the induction heating device and then heated under conditions of a frequency of 1 kHz and a maximum output power of 37 kW to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 50% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A₁ (indicated by character 5 in FIG. 1) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a moving velocity of the pressurizing plunger 9 of 0.07 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting A₁. Aluminum alloy castings A₂ and a were also produced in a similar casting manner using the example A₂ and the comparative example a.

FIGS. 10A and 11A show essential portions of differential thermal analysis thermographs d for the example A₂ and the comparative example a, and FIGS. 9B to 11B show photomicrographs of the metallographic structures of the aluminum alloy castings A₁, A₂ and a.

In the case of the example A₁, a relationship E₁ >E₂ as described above was established between the peak values E₁ and E₂, as shown in FIG. 9A, and from this fact, no defect such as voids on the order of a micron was produced in the aluminum alloy casting A₁, as shown in FIG. 9B.

In the case of the example A₁, the peak value E₁ of the first angled endothermic section e is equal to 9.1 mcal/sec, and the peak value E₂ of the second angled endothermic section f is equal to 4.5 mcal/sec, as shown in FIG. 10A, and hence, E₁ >E₂. Thus, the aluminum alloy casting A₂ is sound, as shown in FIG. 10B.

In the case of the comparative example a, an eutectic melting did not occur (see FIG. 8), as shown in FIG. 6A and hence, only one angled endothermic section was generated. Due to this, the latent heat of the liquid phase is extremely low and as a result, relatively large voids were generated at boundaries between the granular solid phases in the aluminum alloy casting a, as shown in FIG. 11B.

The same description as in the items (B) and (C) in Example 1 is true of the Al-Si based alloy of this example.

Embodiment II

Table 4 shows compositions of examples A₁ to A₄ and comparative examples a₁ to a₅. Each of these examples and comparative examples was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the examples and the comparative examples has a diameter of 50 mm and a length of 65 mm.

                                      TABLE 4                                      __________________________________________________________________________     Al alloy                                                                             Chemical constituents (% by weight)                                      material                                                                             Si Cu Mg Fe Zr Zn Mn V  Ni Ti balance                                    __________________________________________________________________________     Example A.sub.1                                                                      0.17                                                                              10.3                                                                              -- 0.18                                                                              0.18                                                                              -- 0.27                                                                              0.1                                                                               -- 0.05                                                                              Al                                         Example A.sub.2                                                                      5  3  0.32                                                                              0.19                                                                              -- 0.19                                                                              0.09                                                                              -- 0.09                                                                              0.12                                                                              Al                                         Example A.sub.3                                                                      7  0.19                                                                              0.55                                                                              0.19                                                                              -- 0.09                                                                              0.09                                                                              -- -- 0.12                                                                              Al                                         Example A.sub.4                                                                      7  0.19                                                                              0.35                                                                              0.19                                                                              -- 0.09                                                                              0.09                                                                              -- -- 0.19                                                                              Al                                         Comparative                                                                          0.18                                                                              4.6                                                                               0.25                                                                              0.34                                                                              -- 0.09                                                                              0.09                                                                              -- 0.04                                                                              0.22                                                                              Al                                         example a.sub.1                                                                (A204                                                                          material)                                                                      Comparative                                                                          0.17                                                                              6.3                                                                               -- 0.19                                                                              0.08                                                                              -- 0.28                                                                              0.11                                                                              -- 0.05                                                                              Al                                         example a.sub.2                                                                Comparative                                                                          16.5                                                                              4.5                                                                               0.55                                                                              0.4                                                                               -- 0.09                                                                              0.09                                                                              -- -- 0.19                                                                              Al                                         example a.sub.3                                                                (A390                                                                          material)                                                                      Comparative                                                                          0.17                                                                              2.3                                                                               1.55                                                                              1.1                                                                               -- 0.09                                                                              -- -- 1.05                                                                              0.07                                                                              Al                                         example a.sub.4                                                                (2618                                                                          material)                                                                      Comparative                                                                          0.6                                                                               0.27                                                                              1.1                                                                               0.6                                                                               -- 0.24                                                                              0.14                                                                              -- -- 0.14                                                                              Al                                         example a.sub.5                                                                (6061                                                                          material)                                                                      __________________________________________________________________________

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 12. In the differential thermal analysis thermograph d of FIG. 12, a peak value E₁ of a first angled endothermic section e due to an eutectic melting is equal to 2.7 mcal/sec, and a peak value E₂ of a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point is equal to 2.6 mcal/sec. It was confirmed from the above that the ratio E₁ /E₂ of the peak values E₁ and E₂ is equal to 1.04.

On the other hand, in the differential thermal analysis thermograph d, a temperature T₁ of a rising start point g of the first angled endothermic section e is equal to 540° C., and a temperature T₂ of a dropping end point h of the second angled endothermic section f is equal to 636° C. From this, it was confirmed that a difference T₂ -T₁ between the temperatures T₁ and T₂ was equal to 96° C.

Then, the example A₁ was placed into the heating coil in the induction heating device and then heated under conditions of a frequency of 1 kHz and a maximum output power of 37 kW to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 50% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A₁ (indicated by character 5 in FIG. 1) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a moving velocity of the pressurizing plunger 9 of 0.07 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting A₁.

Each of the examples A₂ to A₄ and the comparative examples a₁ to a₅ was subjected to the DSC, and using them, eight aluminum alloy castings A₂ to A₄ and a₁ to a₅ were produced in the same casting process.

Table 5 shows the peak values E₁ and E₂, the ratio E₁ /E₂, the temperature T₁ at the rising start point g, the temperature T₂ at the dropping end point h, the difference T₂ -T₁ between the temperatures T₁ and T₂, the casting temperature for the examples A₁ to A₄ and the comparative examples a₁ to a₅, and the presence or absence of defects in the produced aluminum alloy castings.

                                      TABLE 5                                      __________________________________________________________________________                    Temperature                                                           Peak value                                                                              T.sub.1 at rising                                                                    Temperature  Casting                                                                              Presence of                            Al alloy                                                                             (mcal/sec)                                                                           Ratio                                                                             starting point                                                                       T.sub.2 at dropping                                                                   Difference                                                                           temperature                                                                          absence of defects                     material                                                                             E.sub.1                                                                           E.sub.2                                                                           E.sub.1 /E.sub.2                                                                  (°C.)                                                                         end point (°C.)                                                                T.sub.2 -T.sub.1 (°C.)                                                        (°C.)                                                                         Voids                                                                               Segregation                       __________________________________________________________________________     Example A.sub.1                                                                      2.7                                                                               2.6                                                                               1.04                                                                              540   636    96    615   absence                                                                             absence                           Example A.sub.2                                                                      4.2                                                                               3.5                                                                               1.2                                                                               535   630    95    594   absence                                                                             absence                           Example A.sub.3                                                                      10.4                                                                              5.2                                                                               2  557   629    72    588   absence                                                                             absence                           Example A.sub.4                                                                      7.3                                                                               2.9                                                                               2.5                                                                               566   627    61    590   absence                                                                             absence                           Comparative                                                                          1.8                                                                               9.2                                                                               0.2                                                                               572   657    85    638   presence                                                                            absence                           example a.sub.1                                                                Comparative                                                                          0.6                                                                               5.6                                                                               0.1                                                                               543   648    105   620   presence                                                                            absence                           example a.sub.2                                                                Comparative                                                                          5.9                                                                               0.25                                                                              24 507   650    143   560   presence                                                                            presence                          example a.sub.3                                                                Comparative                                                                          0  10.0                                                                              0  --    --     --    628   presence                                                                            absence                           example a.sub.4                                                                Comparative                                                                          0  12.1                                                                              0  --    --     --    639   presence                                                                            absence                           example a.sub.5                                                                __________________________________________________________________________

FIG. 13 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₁. FIGS. 14A to 16A show essential portions of differential thermal analysis thermographs d for the examples A₂ to A₄, and FIGS. 14B to 16B are photomicrographs showing the metallographic structures of the aluminum alloy castings A₂ to A₄ produced using the examples A₂ to A₄.

As can be seen from Table 5 and FIGS. 14A to 16B, the ratio E₁ /E₂ is in a range of 1<E₁ /E₂ <2.5, and the temperature difference T₂ -T₁ is in a range of 10° C.<T₂ -T₁ <120° C., in each of the examples A₁ to A₄. Therefore, because the liquid phase had a large latent heat, the liquid phase was sufficiently fed around the solid phase in response to the solidification and shrinkage of the solid phase, and the outer periphery 12 of the solid phase was gelled, so that the compatibility between the gelled outer periphery 12 and the liquid phase 13 was improved. Thus, no defects such as voids on the order of a micron were produced in each of these aluminum alloy castings A_(l) to A₄.

FIGS. 17A to 21A show essential portions of differential thermal analysis thermographs d for the examples A₂ to A₄, and FIGS. 17B to 21B are photomicrographs showing the metallographic structures of the aluminum alloy castings a₁ to a₅ produced using the comparative examples a₁ to a₅.

As can be seen from Table 5, both required conditions for the ratio E₁ /E₂ and the temperature difference T₂ -T₁ were not satisfied. Due to this, voids on the order of a micron (black portions) were generated at boundaries in each of the aluminum alloy castings a₁, a₂, a₄ and a₅, as shown in FIGS. 18B, 20B and 21B. In the aluminum alloy casting a₃, the segregation and coalescence of a fine piece-like deep gray primary crystal Si were generated, as shown in FIG. 19B, because the ratio E₁ /E₂ was equal to or greater than 2.5 and moreover, black shrinkage cavities were produced, as shown in FIG. 19B, because the temperature difference T₂ -T₁ was too high. This is also apparent from FIG. 19C.

Then, each of the aluminum alloy castings A₁ to A₄ and a₁ to a₅ was subjected to a T6 treatment under conditions shown in Table

                  TABLE 6                                                          ______________________________________                                                T6 treatment                                                                   Solution treatment                                                                             Aging treatment                                                  Temperature                                                                              Time    Cooling                                                                              Temperature                                                                            Time                                  Al alloy casting                                                                        (°C.)                                                                             (hr)    type  (°C.)                                                                           (hr)                                  ______________________________________                                         A.sub.1  525       5       water 190     18                                    A.sub.2  515       5       cooling                                                                              170     10                                    A.sub.3  540       5             170     5                                     A.sub.4  540       5             170     5                                     a.sub.1  525       6       water 190     5                                     a.sub.2  525       5       cooling                                                                              190     18                                    a.sub.3  490       5             175     8                                     a.sub.4  520       6             185     10                                    a.sub.5  530       5             175     8                                     ______________________________________                                    

For the purpose of carrying out a fatigue test, six test pieces each including a parallel portion having a diameter of 4 mm and a length of 20 mm were fabricated from each of the aluminum alloy castings A₁ to A₄ and a₁ to a₅ resulting from the T6 treatment. These test pieces were subjected to a test with different stress amplitudes using an electric hydraulic fatigue tester to determine the number of repetitions up to the fracture. From this data, the fatigue strength at 10⁷ repetitions was determined. Table 7 shows the results.

                  TABLE 7                                                          ______________________________________                                                        Fatigue strength (MPa)                                          ______________________________________                                         Al alloy casting                                                                              A.sub.1                                                                              120                                                                      A.sub.2                                                                              140                                                                      A.sub.3                                                                              120                                                                      A.sub.4                                                                              120                                                       Al alloy casting                                                                              a.sub.1                                                                              90                                                                       a.sub.2                                                                              95                                                                       a.sub.3                                                                              80                                                                       a.sub.4                                                                              80                                                                       a.sub.5                                                                              70                                                        ______________________________________                                    

FIG. 22 is a graph illustrating the fatigue strengths of the aluminum alloy castings A₁ to A₄ and a₁ to a₅ shown in Table 7.

As can be seen from Table 7 and FIG. 22, each of the aluminum alloy castings A₁ to A₄ produced in the thixocasting process using the examples A₁ to A₄ of the aluminum alloy materials has a fatigue strength higher than those of the aluminum alloy castings a₁ to a₅ produced using the comparative examples a₁ to a₅ of the aluminum alloy materials. Form this fact, it was found that the examples A₁ to A₄ are suitable as thixocasting aluminum alloy materials.

Embodiment III

An aluminum alloy casting 14 shown in FIG. 23 is produced in a thixocasting process using an aluminum alloy material. The aluminum alloy casting 14 includes a medium-thickness portion 14a which is to be connected to a gate 7 of a pressure casting apparatus 1, a thinner portion 14b connected to the medium-thickness portion 14a and having a relatively small volume, and a thicker portion 14c connected to the thinner portion 14b and having a relatively large volume. If the thickness of the thicker portion 14c is represented by t₁, and the thickness of the thinner portion 14b is represented by t₂, a relationship, t₁ =2 t₂ is established.

In the apparatus 1 shown in FIG. 24, a casting forming cavity 4 includes a medium-thickness portion forming zone 4a at a lower portion, a thinner-portion forming zone 4b at a middle portion, and a thick-portion forming zone 4c at an upper portion. The other construction is the same as in the pressure casting apparatus shown in FIG. 1. Therefore, portions or segments corresponding to those in the apparatus shown in FIG. 1 are designated by like reference characters, and the detailed description of them is omitted.

Table 8 shows compositions of examples A₁ to A₃ and comparative examples a₁ and a₂ of aluminum alloy materials. Each of these materials was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the example A₁ and the like has a diameter of 76 mm and a length of 85 mm.

                                      TABLE 8                                      __________________________________________________________________________     Al alloy                                                                             Chemical constituents (% by weight)                                      material                                                                             Si  Cu Mg Fe  Zr Mn V   Ti balance                                       __________________________________________________________________________     Example A.sub.1                                                                      0.17                                                                               10.3                                                                              -- 0.18                                                                               0.18                                                                              0.27                                                                              0.1 0.05                                                                              Al                                            Example A.sub.2                                                                      5.3 2.9                                                                               0.3                                                                               0.12                                                                               -- -- --  0.01                                                                              Al                                            Example A.sub.3                                                                      2.1 9.8                                                                               -- 0.15                                                                               0.15                                                                              0.3                                                                               0.12                                                                               -- Al                                            Comparative                                                                          6.2 3.1                                                                               0.2                                                                               0.72                                                                               -- -- --  0.02                                                                              Al                                            example a.sub.1                                                                Comparative                                                                          2.6 2  4  0.13                                                                               -- -- --  0.03                                                                              Al                                            example a.sub.2                                                                __________________________________________________________________________

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide results shown in FIGS. 25 and 26. In a differential thermal analysis thermograph d shown in each of FIGS. 25 and 26, a peak value E₁ of a first angled endothermic section e due to an eutectic melting is equal to 2.7 mcal/sec, and a peak value E₂ of a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point is equal to 2.6 mcal/sec. From this fact, it was confirmed that E₁ >E₂.

On the other hand, in the differential thermal analysis thermograph d of FIG. 26, a temperature straight line m interconnecting a peak j of the first angled endothermic section e and a temperature graduation of the peak j on an axis k of heating temperature intersects a basic line i which interconnects a rising start point g in the first angled endothermic section e and a dropping end point h in the second angled endothermic section f. A dividing line o bisecting a segment m₁ of the temperature straight line m lying between a first intersection n of the temperature straight line m with the basic line i and the peak j intersects a rising line segment p of the first angled endothermic section e lying between the rising start point g and the peak j. In this condition, the temperature T₃ at a second intersection q of the rising line segment p with the dividing line o is equal to 545° C.

Further, the temperature T₁ at the rising start point g is equal to 540° C., and the temperature T₄ at the peak j is equal to 549° C.

If T₄ -T₁ =.increment.Ta, .increment.Ta is equal to 9° C., and if T₃ -T, =.increment.Tb, .increment.Tb is equal to 5° C. Therefore, in the example S₁, a relationship, .increment.Tb/.increment.Ta=0.55, namely, .increment.Tb/.increment.Ta≦0.68, is established between temperature drop degrees .increment.Ta and .increment.Tb.

The area of the following region FIG. 26 is represented by St: a region which is surrounded by (1) the basic line i which interconnects the rising start point g in the first angled endothermic section e and the dropping end point h in the second angled endothermic section f, (2) a first temperature straight line m corresponding to the above-described temperature straight line which interconnects the peak j of the first angled endothermic section e and the temperature graduation of the peak j on the axis k of heating temperature, and (3) the rising line segment p of the first angled endothermic section lying between the rising start point g and the peak j.

The dividing line o bisecting the segment m₁ of the temperature straight line m lying between the first intersection n of the first temperature straight line m with the basic line i and the peak i also intersects the rising line segment p. Thereupon, the area of the following region is represented by Sm: a region which is surrounded by (1) a second temperature straight line r which interconnects the second intersection q of the dividing line o with the rising line segment p and the temperature graduation of the second intersection q on the heating temperature axis k, (2) a portion P, of the rising line segment p lying between the rising start point g and the second intersection q, and (3) the basic line i.

In the example A₁, a relationship, Sm/St=0.138, namely, Sm/St≦0.365, is established between the areas St and Sm. In measuring the areas St and Sm, a planimeter was used.

In order to establish the relationship, .increment.Tb/.increment.Ta≦0.68 and/or the relationship, Sm/St≦0.365, it is necessary to decrease the amount of Mg and the like which are additional constituents having a low melting point rather than decreasing the amount of Cu, Si and the like which are eutectic constituents, thereby displacing the solid phase line toward the higher temperature side.

Then, the example A₁ was placed into the heating coil in the induction heating device and then heated under conditions of a frequency of 1 kHz and a maximum output power of 37 kW to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 50% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A₁ (indicated by character 5) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a moving velocity of the pressurizing plunger 9 of 0.07 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting 14.

For the results of DSC shown in FIGS. 27 to 31, FIG. 27 corresponds to an example A₂, and FIGS. 28 and 29 correspond to an example A₃, while FIG. 30 corresponds to a comparative example a₁ and FIG. 31 corresponds to a comparative example a₂.

Using these examples A₂ and A₃ and comparative examples a₁ and a₂, four aluminum alloy castings were produced in the same casting process as those described above.

Table 9 shows the peak values E₁ and E₂, the ratio E₁ /E₂, the temperature drop ranges .increment.Ta and .increment.Tb, the ratio .increment.Tb/.increment.Ta, the ratio Sm/St of the areas St and Sm for the examples A₁ to A₃ and the comparative examples a₁ and a₂, and the presence or absence of defects in the produced aluminum alloy castings produced from the examples A₁ to A₃ and the comparative examples a₁ and a₂.

                                      TABLE 9                                      __________________________________________________________________________                     Temperature drop                                               Al alloy                                                                             Peak value (mcal/sec)                                                                    ranges (°C.)                                                                    Ratio                                                                               Ratio                                                                              Presence or                                   material                                                                             E.sub.1                                                                             E.sub.2                                                                             ΔTa                                                                          ΔTb                                                                          ΔTb/ΔTa                                                                 Sm/St                                                                              absence of defects                            __________________________________________________________________________     Example A.sub.1                                                                      2.7  2.6   9   5  0.55 0.138                                                                              absence                                       Example A.sub.2                                                                      4.9  4.2  35  24  0.68 0.364                                                                              absence                                       Example A.sub.3                                                                      3.7  1.3   6   3  0.50 0.365                                                                              absence                                       Comparative                                                                          4.4  1.4  45  36  0.80 0.438                                                                              presence                                      example a.sub.1                                                                Comparative                                                                          3.2  2.9  56  45  0.80 0.709                                                                              presence                                      example a.sub.2                                                                __________________________________________________________________________

FIGS. 32 to 34B are photomicrographs each showing the metallographic structure of the thicker portion 14c of the aluminum alloy casting 14. FIG. 32 corresponds to the case where the example A₁ is used; FIG. 33 corresponds to the case where the example A₂ is used; and FIGS. 34A and 34B correspond to the case where the comparative example a₁ is used.

As can be seen from FIGS. 32 and 33, in the case of the aluminum alloy castings produced using the examples A₁ and A₂, there is no defect such as voids on the order of a micron generated in the thicker portion 14c of the aluminum alloy casting 14, because the relationship E₁ >E₂ is established between the peak values E₁ and E₂, and the relationship .increment.Tb/.increment.Ta≦0.68 is established between the temperature drop ranges .increment.Tb and .increment.Ta. In these cases, the relationship, Sm/St≦0.365 is also established between the areas St and Sm.

If the latent heat of the liquid phase is decreased to 1/2 of the maximum released latent heat, namely, to E₁ /2 in the course of the solidification of the semi-molten alloy material, the liquid phase is thereafter gelled, so that the viscosity is gradually increased. This causes a particularly degraded feedability of the liquid phase around the solid phase in the thicker portion 14c of the aluminum alloy casting and hence, voids on the order of a micron are liable to be generated in the thicker portion 14c. This tendency is doubled, because the thinner portion forming zone 4b is located short of the thicker portion forming zone 4c, as shown in FIG. 24.

Since .increment.Ta represents the temperature drop range required for the liquid phase to be solidified, and .increment.Tb represents the temperature drop range required for the gelled phase produced from the liquid phase to be solidified, if the ratio .increment.Tb/.increment.Ta is set in the range of .increment.Tb/.increment.Ta≦0.68, the temperature range required for the gelled phase to be solidified is narrowed, and on the other hand, the temperature range required for the gelled phase to be produced from the liquid phase is widened. Thus, it is possible to improve the feedability of the liquid phase around the solid phase in the thicker portion 14c of the aluminum alloy casting 14 to prevent the generation of voids on the order of a micron.

Since the area St represents the released latent heat amount required for the liquid phase to be solidified, and the area Sm represents the released latent heat amount required for the gelled phase produced from the liquid phase to be solidified, if the ratio Sm/St is set in the range of Sm/St≦0.365, the released latent heat amount required for the gelled phase to be solidified is decreased and on the other hand, the released latent heat amount permitting the gelled phase to be produced from the liquid phase is increased. Thus, it is possible to improve the feedability of the liquid phase around the solid phase in the thicker portion 14c of the aluminum alloy casting 14 to prevent the generation of voids on the order of a micron.

In the case of the aluminum alloy casting 14 produced using the comparative example a₁ shown in FIGS. 34A and 34B, the relationship E₁ >E₂ is established, but the relationship .increment.Tb/.increment.Ta≦0.68 and/or the relationship Sm/St ≦0.365 are not established and hence, the thicker portion 14c has voids on the order of a micron generated therein.

In any of the aluminum alloy castings, the thinner portion 14b has no defects generated therein.

FIG. 35 shows the relationship between the ratio .increment.Tb/.increment.Ta and the defect area proportion in the thicker portion 14c. It can be seen from FIG. 35 that defects are generated when .increment.Tb/.increment.Ta>0.68.

FIG. 36 shows the relationship between the ratio Sm/St and the defect area proportion in the thicker portion 14c. It can be seen from FIG. 36 that defects are generated when Sm/St>0.365.

The defect area proportion in the thicker portion was measured using an image resolving device.

Embodiment IV

Table 10 shows compositions of examples A₁ to A₇ and comparative examples a₁ and a₂. The example A₁ is an Al-Si-CU based alloy; the examples A₂ to A₄ are Al-Si-Mg based alloys; the examples A₅ to A₇ and the comparative example a₁ are Al-Cu based alloys; and the comparative example a₂ is an Al-Zn-Mg based alloy. Each of the examples and comparative examples was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the examples and the comparative examples has a diameter of 50 mm and a length of 65 mm.

                                      TABLE 10                                     __________________________________________________________________________     Al alloy                                                                             Chemical constituents (% by weight)                                      material                                                                             Si Cu Mg Fe Zr Zn Mn V  Ti balance                                       __________________________________________________________________________     Example A.sub.1                                                                      6.8                                                                               3.9                                                                               0.1                                                                               0.1                                                                               -- 0.1                                                                               0.2                                                                               -- 0.15                                                                              Al                                            Example A.sub.2                                                                      7  <0.2                                                                              0.35                                                                              <0.2                                                                              -- <0.1                                                                              <0.1                                                                              -- <0.2                                                                              Al                                            Example A.sub.3                                                                      7  <0.2                                                                              0.55                                                                              <0.2                                                                              -- <0.1                                                                              <0.1                                                                              -- 0.12                                                                              Al                                            Examp1e A.sub.4                                                                      5  3  0.32                                                                              <0.2                                                                              -- <0.2                                                                              <0.1                                                                              -- 0.13                                                                              Al                                            Example A.sub.5                                                                      0.15                                                                              12.2                                                                              -- 0.18                                                                              0.15                                                                              -- 0.3                                                                               0.1                                                                               0.05                                                                              Al                                            Example A.sub.6                                                                      0.18                                                                              8.2                                                                               -- 0.18                                                                              0.2                                                                               -- 0.25                                                                              0.1                                                                               0.05                                                                              Al                                            Example A.sub.7                                                                      0.17                                                                              10.3                                                                              -- 0.18                                                                              0.18                                                                              -- 0.27                                                                              0.1                                                                               0.05                                                                              Al                                            Comparative                                                                          0.11                                                                              1.9                                                                               1.4                                                                               1.3                                                                               -- -- 1.3                                                                               -- 0.1                                                                               Al                                            example a.sub.1                                                                Comparative                                                                          0.4                                                                               <0.1                                                                              7.8                                                                               <0.3                                                                              -- 28.8                                                                              0.3                                                                               -- 0.2                                                                               Al                                            example a.sub.2                                                                __________________________________________________________________________

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 37. In the differential thermal analysis thermograph d shown in FIG. 37, there are a first angled endothermic section e due to an eutectic melting, and a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point. In this case, the temperature T₄ at a peak j of the first angled endothermic section e is equal to 559° C., and the temperature at a peak of the second angled endothermic section f is equal to 579° C. Therefore, it is found that the difference T₅ -T₄ between the temperatures T₄ and T₅ is equal to 20° C.

Then, the example A₁ was placed into a heating coil in an induction heating device and then heated under conditions of a frequency of 1 kHz, a maximum output power of 30 kW and a heating time of 7 minutes to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 40% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A₁ (indicated by character 5) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a moving velocity of the pressurizing plunger 9 of 0.2 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting A₁.

Each of the examples A₂ to A₇ and the comparative examples a₁ and a₂ was also subjected to DSC. Further, eight aluminum alloy castings A₂ to A₇, a₁ and a₂ were produced in the same casting process using the examples A₂ to A₇ and the comparative examples a₁ and a₂. In this case, the heating time was 7 minutes as described above. For comparison, an aluminum alloy casting a₃ was produced for a heating time set at 20 minutes in an electric resistance furnace.

Then, each of the aluminum alloy castings A₁ to A₇ and a₁ to a₃ was subjected to a T6 treatment under conditions shown in Table

                  TABLE 11                                                         ______________________________________                                         T6 treatment                                                                   Solution treatment    Aging treatment                                          Al alloy                                                                             Temperature                                                                               Time    Cooling                                                                               Temperature                                                                             Time                                  casting                                                                              (°C.)                                                                              (hr)    type   (°C.)                                                                            (hr)                                  ______________________________________                                         A.sub.1                                                                              500        5       water  160       5                                    A.sub.2                                                                              540                cooling                                                                               170                                            A.sub.3                                                                        A.sub.4                                                                              515                                10                                    A.sub.5                                                                              525                       190      18                                    A.sub.6                                                                        A.sub.7                                                                        a.sub.1                                                                              525        8       water  160       9                                    a.sub.2                  cooling                                               a.sub.3                                                                              465                       125      28                                    ______________________________________                                    

For the purpose of carrying out a fatigue test, eight test pieces each including a parallel portion having a diameter of 4 mm and a length 20 mm were fabricated from each of the aluminum alloy castings A₁ to A₇ and a₁ to a₃ resulting from the T6 treatment. Each of the test pieces was subjected to a test with different stress amplitudes at room temperature using an electric hydraulic fatigue tester to determine the number of repetitions up to the fracture. From this data, the fatigue strength S₁ at 10⁷ repetitions was determined. Table 7 shows the results.

FIGS. 38 to 43, 44 and 45 show differential thermal analysis thermographs for the examples A₂ to A₇ and the comparative examples a₁ and a₂.

Table 12 shows the temperatures at the peaks j of the first and second angled endothermic sections e and f, the difference T₅ -T₄ between the temperatures T₄ and T₅, the heating time and the casting temperature for the examples A₁ to A₇ and the comparative examples a₁ and a₂, as well as the presence or absence of voids, the presence or absence of the coalescence of the primary crystal α-Al and the fatigue strength S₁ for the aluminum alloy castings A₁ to A₇ and a₁ to a₃.

                                      TABLE 12                                     __________________________________________________________________________     Al alloy material                                                              Al  Temperature  Heating                                                                             Casting       Presence or absence                                                                     Fatigue                           alloy                                                                              at peaks                                                                               Difference                                                                          time temperature                                                                          Presence or                                                                            of coalescence of                                                                       strength                          material                                                                           T.sub.4 (°C.)                                                               T.sub.5 (°C.)                                                               T.sub.5 -T.sub.4 ( )                                                                (minute)                                                                            (°C.)                                                                         absence of voids                                                                       primary crystal α-Al                                                              S.sub.1 (MPa)                     __________________________________________________________________________     A.sub.1                                                                            559 579 20   7    572   absence absence  120                               A.sub.2                                                                            580 619 39        590                    120                               A.sub.3                                                                            578 618 40        588                    120                               A.sub.4                                                                            571 624 53        594                    140                               A.sub.5                                                                            549 623 74        595                    130                               A.sub.6                                                                            547 626 79        612                    130                               A.sub.7                                                                            549 629 80        605                    130                               a.sub.1                                                                            630 640 10   7    586   presence                                                                               absence   80                               a.sub.3                                                                            630 640 10   20   586   absence presence  90                               a.sub.2                                                                            481 586 105  7    573   presence                                                                               absence   60                               __________________________________________________________________________

FIG. 46 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₇. FIGS. 47A, 48 and 49 are photomicrographs showing the metallographic structures of the aluminum alloy castings a₁, a₃ and a₂, respectively.

As can be seen from FIGS. 37 to 43 and 46 and Table 12, the aluminum alloy castings A₁ to A₇ were produced from the examples A₁ to A₇ with the difference T₅ -T₄ between the temperatures set in a range of 20° C.≦T₅ -T₄ ≦80° C., and the variation in solid phase proportion relative to the variation in heating temperature was blunted. Thus, it is possible to increase the temperature rising rate and heat the examples A₁ to A₇ to the casting temperature within a short time of 7 minutes, thereby preventing the coalescence of the primary crystal α-Al.

In addition, the diffusion of the primary crystal α-Al is actively effected as a result of widening of the temperature range permitting the solid and liquid phases to coexist, and therefore, the fine spheroidization and the uniform dispersion of the primary crystal α-Al are promoted, and the generation of shrinkage cavities is also avoided.

From this fact, each of the aluminum alloy castings A₁ to A₇ has a sound casting quality and an excellent fatigue strength.

In the case of the aluminum alloy casting a, shown in FIGS. 47A and 47B, voids were generated due to the inclusion of air occurring during pouring of the material, because the difference T₅ -T₄ between the temperatures in the comparative example a₁ was lower than 20° C., and the heating time was as short as 7 minutes.

In the case of the aluminum alloy casting a₃ shown in FIG. 48, the primary crystal α-Al (massive portion) was coalesced, because the heating time for the comparative example a₃ was as long as 20 minutes.

In the case of the aluminum alloy casting a₂ shown in FIG. 49, shrinkage cavities and thus, voids (black portions), were generated during solidification, because the difference T₅ -T₄ between the temperatures in the comparative example a₃ was higher than 80° C.

For comparison, using the examples A₁ to A₇ and the comparative examples a₁ and a₂, aluminum alloy extrusions A₁ to A₇, a₁ and a₂ were produced by an extruding process under conditions of an extruding temperature of 350° to 420° C., a container temperature of 300° C., a die temperature of 250° C., a die bore diameter of 35 mm and an extrusion ratio of 4.7.

Each of the aluminum alloy extrusions A₁ to A₇, a₁ and a₂ was subjected to a T6 treatment under the same conditions as for the aluminum alloy castings A₁ to A₇, a₁ and a₂ (see Table 11). Then, test pieces similar to those described above were fabricated from the aluminum alloy extrusions A₁ to A₇, a₁ and a₂ and subjected to a fatigue test similar to that described above to determine the fatigue strength S₂ at 10⁷ repetitions.

Table 13 shows the strength ratio S₁ /S₂ of the fatigue strength S₁ of each of the aluminum alloy castings A₁ to A₇, a₁, a₃ and a₂ to the fatigue strength S₂ of each of the aluminum alloy extrusions A₁ to A₇, a₁, a₃ and a₂. In Table 13, the aluminum alloy extrusion a₃ is the same as the aluminum alloy extrusion a₁.

                  TABLE 13                                                         ______________________________________                                         Al alloy extrudate                                                                        Fatigue strength S.sub.2 (MPa)                                                                 Strength ratio S.sub.1 /S.sub.2                     ______________________________________                                         A.sub.1    120             1.0                                                 A.sub.2    120             1.0                                                 A.sub.3    120             1.0                                                 A.sub.4    140             1.0                                                 A.sub.5    130             1.0                                                 A.sub.6    130             1.0                                                 A.sub.7    130             1.0                                                 a.sub.1    100             0.8                                                 a.sub.3    100             0.9                                                 a.sub.2    110             0.5                                                 ______________________________________                                    

FIG. 50 is a graph illustrating the relationship between the difference T₅ -T₄ between the temperatures and the strength ratio S₁ /S₂ for the examples A₁ to A₇ and the comparative examples a₁ and a₂. In FIG. 50, points A₁ to A₇, a₁ and a₂ correspond to the cases where the examples A₁ to A₇ and the comparative examples a₁ and a₂ were used, respectively. Point a₃ corresponds to the case where the heating time for the comparative example a, was 20 minutes.

As can be seen from FIG. 50 and Table 13, if the examples A₁ to A₇ are used, the aluminum alloy castings A₁ to A₇ having a fatigue strength equivalent to those of the aluminum alloy extrusions A₁ to A₇ can be produced. From this, it can be also seen that the difference T₅ -T₄ between the temperatures may be set in the range of 20° C.≦T₅ -T₄ ≦80° C.

Embodiment V

Table 14 shows the compositions of examples A₁ to A₄ and comparative examples a₁ to a₃ of aluminum alloy materials. Each of the examples and the comparative examples was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the examples and the comparative examples has a diameter of 50 mm and a length of 65 mm.

                                      TABLE 14                                     __________________________________________________________________________     Al alloy                                                                             Chemical constituents (% by weight)                                      material                                                                             Si Cu Mg Fe Zr Zn Mn V  Ti Balance                                       __________________________________________________________________________     Example A.sub.1                                                                      0.15                                                                              12.2                                                                              -- 0.18                                                                              0.15                                                                              -- 0.3                                                                               0.1                                                                               0.05                                                                              Al                                            Example A.sub.2                                                                      0.17                                                                              10.3                                                                              -- 0.18                                                                              0.18                                                                              -- 0.27                                                                              0.1                                                                               0.05                                                                              Al                                            Example A.sub.3                                                                      0.18                                                                              8.2                                                                               -- 0.18                                                                              0.2                                                                               -- 0.25                                                                              0.1                                                                               0.05                                                                              Al                                            Example A.sub.4                                                                      0.1                                                                               18 -- 0.18                                                                              0.15                                                                              -- 0.3                                                                               0.1                                                                               0.05                                                                              Al                                            Comparative                                                                          7  <0.2                                                                              0.55                                                                              <0.2                                                                              -- <0.1                                                                              <0.1                                                                              -- 0.12                                                                              Al                                            example a.sub.1                                                                Comparative                                                                          5  3  0.33                                                                              <0.2                                                                              -- <0.2                                                                              <0.1                                                                              -- 0.13                                                                              Al                                            example a.sub.2                                                                Comparative                                                                          7  <0.2                                                                              0.35                                                                              <0.2                                                                              -- <0.1                                                                              <0.1                                                                              -- <0.2                                                                              Al                                            example a.sub.3                                                                __________________________________________________________________________

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 51. In a differential thermal analysis thermograph d shown in FIG. 51, there are a first angled endothermic section e due to an eutectic melting, and a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point. The second angled endothermic section f has, between a peak v thereof and a dropping end point u of the first angled endothermic section e, a gentle-sloping portion x extending along a basic line i which interconnects a rising start point g of the first angled endothermic section e and a dropping end point h of the second angled endothermic section f.

The temperature T₁ at the rising start point g in the first angled endothermic section e is equal to 537° C., and the temperature T₄ at the peak j is equal to 549° C. The temperature T₅ at the peak v in the second angled endothermic section f is equal to 623° C., and the temperature T₂ at the dropping end point h is equal to 629° C. Therefore, a solid/liquid phase-coexisting temperature range Tc (=T₂ -T₁) between the rising start point g of the first angled endothermic section e and the dropping end point h of the second angled endothermic section f is equal to 92° C., and a peak-peak temperature range Td (=T₅ -T₄) between the peaks j and v of the first and second angled endothermic sections e and f respectively, is equal to 74° C. In this case, the proportion Rt (Td/Tc)×100 of the peak-peak temperature range Td in the solid/liquid phase-coexisting temperature range Tc is 80%.

Then, the example A₁ was placed into the heating coil in the induction heating device and then rapidly heated for 4 minutes up to a casting temperature T₆ (=595° C.) under conditions of a frequency of 1 kHz and a maximum output power of 25 kW to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 40% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example A₁ (indicated by character 5) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of the casting temperature of 595° C., a moving velocity of the pressurizing plunger 9 of 0.2 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting A₁.

The examples A₂ to A₄ and the comparative examples a₁ to a₃ were also subjected to DSC, and further, using these examples and comparative examples, six aluminum alloy castings A₂ to A₄ and a₁ to a₃ were produced in the same casting process.

FIGS. 52 to 54 show differential thermal analysis thermographs d for the examples A₂ to A₄, and FIGS. 55 to 57 show differential thermal analysis thermographs d for the comparative examples a₁ to a₃. It can be seen by the comparison of FIGS. 52 to 54 with FIGS. 55 to 57 that for each of the examples A₂ to A₄ shown in FIGS. 52 to 54, a gentle-sloping portion x exists, but for each of the comparative examples a₁ to a₃ shown in FIGS. 55 to 57, no gentle-sloping portion x exists.

Table 15 shows the solid/liquid-coexisting temperature Tc, the peak-peak temperature range Td, and the proportion Rt of the peak-peak temperature range Td in the solid/liquid phase-coexisting temperature range Tc for the examples A₁ to A₄ and the comparative examples a₁ to a₃, and the metallographic structure of the corresponding aluminum alloy castings A₁ to A₄ and a₁ to a₃.

                  TABLE 15                                                         ______________________________________                                                      Solid/liquid                                                                               Peak-peak                                                          phase-coexisting                                                                           temperature                                                        temperature range Td  Proportion                                  Al alloy material                                                                           range Tc (°C.)                                                                      (°C.)                                                                             Rt (%)                                      ______________________________________                                         Example A.sub.1 (casting A.sub.1)                                                           92          74        80                                          Example A.sub.2 (casting A.sub.2)                                                           96          80        83                                          Example A.sub.3 (casting A.sub.3)                                                           94          79        84                                          Example A.sub.4 (casting A.sub.4)                                                           59          41        69                                          Comparative example a.sub.1                                                                 72          40        56                                          (casting a.sub.1)                                                              Comparative example a.sub.2                                                                 95          53        56                                          (casting a.sub.2)                                                              Comparative example a.sub.3                                                                 61          39        64                                          (casting a.sub.3)                                                              ______________________________________                                    

FIG. 58 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₁, and FIG. 59 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₂. It can be seen from FIGS. 58 and 59 that each of the examples A₁ and A₂ has an extremely uniform metallographic structure, notwithstanding it was produced by rapidly heating each of the examples A₁ and A₂.

FIG. 60 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₄. As apparent from FIG. 60, a somewhat larger α-Al solidified phase is observed in the aluminum alloy casting A₄, but the metallographic structure of the aluminum alloy casting A₄ is substantially uniform.

FIG. 61 is a photomicrograph showing the metallographic structure of the aluminum alloy casting a₁. As apparent from FIG. 61, the distribution of the liquid phase in the aluminum alloy casting a₁ was not uniformized due to the rapid heating of the comparative example a₁ and as a result, the α-Al solidified phases were agglomerated to provide the non-uniform metallographic structure. The same is true for the aluminum alloy castings a₂ and a₃.

The following can be seen from the above results. In order to produce an aluminum alloy casting having a uniform metallographic structure, it is required that the second angled endothermic section f have a gentle-sloping portion x in the differential thermal analysis thermograph d, as described above, and the proportion Rt of the peak-peak temperature range Td in the solid/liquid coexisting temperature Tc be in the range of Rt≧69%, as in the examples A₁ to A₄.

In the examples A₁ to A₄ having such a thermal characteristic, the temperature range between the dropping end point of the first angled endothermic section e and the peak v of the second angled endothermic section f is wide and therefore, the gellation of the outer periphery of the solid phase is promoted. This improves the compatibility between the solid and liquid phases and hence, it is possible to avoid a disadvantage of the generation of voids on the order of a micron at a boundary between the solid and liquid phases.

For comparison, using the examples A₁ to A₄ and the comparative examples a₁ to a₃, seven aluminum alloy castings B₁ to B₄ and b₁ to b₃ corresponding to the examples A₁ to A₄ and the comparative examples a₁ to a₃, respectively were produced in the same casting process under the same conditions, except that the examples A₁ to A₄ and the comparative examples a₁ to a₃ were heated (in a normally heating manner) up to the casting temperature T₆ under conditions of a frequency of 1 kHz, a maximum power output of 12 kW and a heating time of 7 minutes.

The metallographic structure of each of the aluminum alloy castings B₁ to B₄ and b₁ to b₃ was observed by a microscope, and the result showed that the metallographic structure was uniform.

Then, each of the aluminum alloy castings A₁ to A₄, B₁ to B₄, a₁ to a₃ and b₁ to b₃ was subjected to a T6 treatment under conditions shown in Table

                  TABLE 16                                                         ______________________________________                                                T6 treatment                                                                   Solution treatment                                                                             Aging treatment                                                  Temperature                                                                              Time    Cooling                                                                              Temperature                                                                            Time                                  Al alloy casting                                                                        (°C.)                                                                             (hr)    type  (°C.)                                                                           (hr)                                  ______________________________________                                         A.sub.1, B.sub.1                                                                        525       5       Water 190     18                                    A.sub.2, B.sub.2           cooling                                             A.sub.3, B.sub.3                                                               A.sub.4, B.sub.4                                                               a.sub.1, b.sub.1                                                                        540       5       Water 170      5                                    a.sub.2, b.sub.2                                                                        515               cooling       10                                    a.sub.3, b.sub.3                                                                        540                              5                                    ______________________________________                                    

For the purpose of carrying out a fatigue test, ten test pieces each including a parallel portion having a diameter of 4 mm and a length of 20 mm were fabricated from each of the aluminum alloy castings A₁ to A₄, B₁ to B₄, a₁ to a₃ and b₁ to b₃ resulting from the T6 treatment. These test pieces were subjected to a test with different stress amplitudes using, an electric hydraulic fatigue tester to determine the number of repetitions up to the fracture. From this data, the fatigue strength at the number of 10⁷ repetitions was determined. Table 17 shows the results. In the strength ratio heading shown in Table 17, "rapid heating/normal heating" means that the fatigue strength value, for example, for the aluminum alloy A₁ produced through the rapid heating, is divided by the fatigue strength value for the aluminum alloy B₁ produced through the normal heating.

                  TABLE 17                                                         ______________________________________                                         Al alloy                                                                             Fatigue strength (Mpa)                                                                           Fatigue strength ratio of rapid                        casting                                                                              Rapid heating                                                                             Normal heating                                                                            heating/normal heating                             ______________________________________                                         A.sub.1                                                                              130        --         1.0                                                B.sub.1                                                                              --         130                                                           A.sub.2                                                                              130        --         1.0                                                B.sub.2                                                                              --         130                                                           A.sub.3                                                                              130        --         1.0                                                B.sub.3                                                                              --         130                                                           A.sub.4                                                                              100        --         0.95                                               B.sub.4                                                                              --         105                                                           a.sub.1                                                                              100        --         0.8                                                b.sub.1                                                                              --         120                                                           a.sub.2                                                                              110        --         0.8                                                b.sub.2                                                                              --         140                                                           a.sub.3                                                                              100        --         0.8                                                b.sub.3                                                                              --         120                                                           ______________________________________                                    

FIG. 62 is a graph illustrating the relationship between the proportion Rt of the peak-peak temperature range Td in the solid/liquid-coexisting temperature Tc and the fatigue strength ratio for the aluminum alloy castings A₁ and B₂ and the like shown in Tables 15 and 17. In FIG. 62, points A₁ to A₄ correspond to the cases where the examples A₁ to A₄ were used, and points a₁ to a₃ correspond to the cases where the comparative examples a₁ to a₃ were used.

As apparent from Tables 15 and 17 and FIG. 62, each of the aluminum alloy castings A₁ to A₄ produced using the examples A₁ to A₄ through the rapid heating has a fatigue strength equivalent to those of the aluminum alloy castings B₁ to B₄ produced through the normal heating. This is because each of the aluminum alloy castings A₁ to A₄ has a uniform metallographic structure, as do the aluminum alloy castings B₁ to B₄. The uniformization of the metallographic structure is due to the fact that the proportion Rt in the examples A₁ and the like is equal to or greater than 69% (Rt≧69%).

On the other hand, each of the aluminum alloy castings a₁ to a₃ produced using the comparative examples a₁ to a₃ through the rapid heating has a low fatigue strength, as compared with the aluminum alloy castings b₁ to b₃ produced through the normal heating. This is because the metallographic structure of the aluminum alloy castings a₁ to a₃ is non-uniform. The non-uniformization of the metallographic structure is due to the fact that the proportion Rt in the comparative examples a₁ and the like is smaller than 69% (Rt<69%).

Embodiment VI

Table 18 shows the compositions of aluminum alloy materials A₁ to A₅. Each of the examples A₁ to A₅ was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, a spheroidizing treatment of a primary crystal α-Al was performed. Each of the examples A₁ to A₅ has a diameter of 50 mm and a length of 65 mm.

                  TABLE 18                                                         ______________________________________                                                 Chemical constituents (% by weight)                                    Al alloy material                                                                        Si     Cu      Mg    Fe    Ti    balance                             ______________________________________                                         A.sub.1   0.15   12.2    --    0.18  0.15  Al                                  A.sub.2   5.3    2.9     0.3   0.12  0.01  Al                                  A.sub.3   7      <0.2    0.45  <0.2  <0.2  Al                                  A.sub.4   0.12   10.3    --    0.1   0.05  Al                                  A.sub.5   0.18   8.2     --    0.18  0.05  Al                                  ______________________________________                                    

The example A₁ was subjected to a differential scanning calorimetry (DSC) to provide the result shown in FIG. 63. In the differential thermal analysis thermograph d of FIG. 63, there are a first angled endothermic section e due to an eutectic melting, and a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point. The temperature T₁ at a rising start point g in the first angled endothermic section e is equal to 537° C., and the temperature T₇ at a dropping end point u is equal to 561° C. The temperature T₅ in a peak v in the second angled endothermic section f is equal to 624° C.

Then, the example A₁ was placed in a upstanding fashion into the heating coil in the induction heating device and then heated up to a casting temperature T₆ of 595° C. under conditions shown in FIG. 64 to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. When the temperature of the example A₁ was in a range of 30° C. to the temperature T₁ of 537° C. at the rising start point, the frequency was set at 1 kHz, and the power output was set at 20 kW. When the temperature of the example A₁ was in a range of the temperature T₁ of 537° C. at the rising start point g to the temperature T₇ of 561° C. at the dropping end point u, the frequency was set at 1 kHz, and the power output was set at 15 kW. Further, when the temperature of the example A₁ was in a range of the temperature T₇ of 561° C. at the dropping end point u to the casting temperature T₆, the frequency was set at 1 kHz, and the power output was set at 3 kW. Thus, the average temperature rising rate α for the example A₁ between the rising start point g and the dropping end point u was controlled to 0.23° C./sec, while the average temperature rising rate β for the example A₁ between the dropping end point u and the casting temperature T₆ was controlled to 0.18° C./sec. In this case, the solid phase proportion in the example A₁ was in a range of 40% (inclusive) to 60% (inclusive). The measurement of the temperature of the example A₁ was performed by inserting a thermocouple into a measuring hole opened at the center of the lower surface of the example A₁.

Thereafter, the semi-molten example A₁ (indicated by character 5) was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of a casting temperature T₆ of 595° C., a moving velocity of the pressurizing plunger 9 of 0.2 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the example A₁ fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the example A₁ was solidified under the pressurization to provide an aluminum alloy casting A₁₁. Using the example A₁, an aluminum alloy casting A₁₂ was also produced in a similar casting manner, except that the average temperature rising rates α and β were changed.

The examples A₂ to A₅ were also subjected to DSC, and further, using these examples A₂ to A₅, eight aluminum alloy castings A₂₁, A₃₁, A₄₁, A₅₁, A₂₂, A₃₂, A₄₂ and A₅₂ were produced by the same casting process. FIGS. 65 to 68 show differential thermal analysis thermographs d for the examples A₂ to A₅, respectively.

Each of the aluminum alloy castings A₁₁ to A₅₁ and A₁₂ to A₅₂ was observed by a microscope to determine the presence or absence of on the order of a micron and the size of α-Al.

FIG. 69A is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₁₁, in which a relatively large granular portion is α-Al. It can be seen from FIG. 69A that voids as described were not generated in the aluminum alloy casting A₁₁.

FIG. 69B shows how to measure the size of an α-Al phase. In measuring the size of the α-Al phase, two straight lines L₁ and L₂ intersecting each other are first drawn so as to traverse a plurality of α-Al phases and then, the length of a line segment La of each of the straight lines L₁ and L₂ in each of the α-Al phases is measured, whereby an average value of the lengths is determined. Thus, the average value of the lengths of all the line segments La of one the straight line L₁ is (25 μm+35 μm+67.5 μm)/3=42.5 μm, and the average value of the lengths of all the line segments La of the other straight line L₂ is (60 μm+40 μm+75 μm)/3≅58.3 μm. Thereafter, an average value of such average values is further determined and defined as the size of the α-Al phase. Thus, the size of the α-Al phase is (42.5 μm+58.3 μm)/2≅50 μm.

FIG. 70 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₁₂, in which a relatively large portion is an α-Al phase. It can be seen that voids as described above were generated in the aluminum alloy casting A₁₂, as also shown in FIG. 70B. In this case, an average value of the lengths of all the line segments La of the straight line L₁ is (62.5 μm+60 μm)/2=61.25 μm, and an average value of the lengths of all the line segments La of the other straight line L₂ is (40 μm+67.5 μm+87.5 μm)/3≅65 m. Therefore, the size of the α-Al phase is (61.25 μm+65 μm)/2≅63 μm.

Each of the aluminum alloy castings A₁₁ to A₅ and A₁₂ to A₅₂ was subjected to a T6 treatment. Then, for the purpose of carrying out a fatigue test, ten test pieces each including a parallel portion having a diameter of 4 mm and a length 20 mm were fabricated from each of the aluminum alloy castings A₁₁ to A₅₁ and A₁₂ to A₅₂ resulting from the T6 treatment. Each of the test pieces was subjected to a test with different stress amplitudes using an electric hydraulic fatigue tester to determine the number of repetitions up to the fracture. From this data, the fatigue strength at 10⁷ repetitions was determined.

Table 19 shows the average temperature rising rates α and β, the ratio α/β, the size of the α-Al phase, the presence or absence of voids and the fatigue strength for the aluminum alloy castings A₁₁ to A₅₁ and A₁₂ to A₅₂.

                                      TABLE 19                                     __________________________________________________________________________         Average                                                                              Average                                                                  temperature                                                                          temperature              Fatigue                                     Al alloy                                                                           rising rate                                                                          rising rate                                                                               Size of α-Al                                                                   Presence and                                                                           strength                                    casting                                                                            α (°C./sec)                                                             β (°C./sec)                                                              Ratio α/β                                                                (μm)                                                                              absence of voids                                                                       (MPa)                                       __________________________________________________________________________     A.sub.11                                                                           0.23  0.18  1.3   50   absence 130                                         A.sub.12                                                                           0.18  0.20  0.9   63   presence                                                                                90                                         A.sub.21                                                                           0.31  0.29  1.1  150   absence 140                                         A.sub.22                                                                           0.30  0.32  0.9  150   presence                                                                               100                                         A.sub.31                                                                           0.34  0.12  2.8  110   absence 110                                         A.sub.32                                                                           0.14  0.18  0.8  129   presence                                                                                90                                         A.sub.41                                                                           1.40  0.26  5.4  120   absence 130                                         A.sub.42                                                                           0.26  0.32  0.8  140   presence                                                                                90                                         A.sub.51                                                                           0.58  0.19  3.1  140   absence 130                                         A.sub.52                                                                           0.30  0.36  0.8  150   presence                                                                                90                                         __________________________________________________________________________

FIG. 71 is a graph illustrating the relationship between the ratio α/β and the fatigue strength for the aluminum alloy castings A₁₁ to A₅₁ and A₁₂ to A₅₂ as taken from Table 19.

As apparent from Table 19 and FIG. 71, each of the aluminum alloy castings A₁₁ to A₅₁ produced with the ratio α/β between the temperature rising rates α and β set in a range of α/β>1, has an enhanced fatigue strength, as compared with the corresponding aluminum alloy castings A₁₂ to A₅₂.

As can be seen by comparing of the aluminum alloy castings A₁₁ and A₁₂ ; A₃₁ and A₃₂ ; A₄₁ and A₄₂ ; A₅₁ and A₅₂ with each other, the size of the α-Al phase can be maintained smaller, when the average temperature rate α is higher. The aluminum alloy castings A₂₁ and A₂₂ have the same size of the α-Al phase, because of the substantially equal average temperature rising rate.

As can be also seen by comparing the aluminum alloy castings A₁₁ and A₁₂ ; A₃₁ and A₃₂ ; A₄₁ and A₄₂ ; A₅₁ and A₅₂ with each other, it is possible to prevent the generation of voids on the order of a micron by setting the average temperature rising rates α and β, so that the average temperature rising rate α is lower than the average temperature rising rate β. This is attributable to the fact that the profile of temperature of the liquid phase was uniformized, and that the outer periphery of the solid phase was gelled to provide an improved compatibility between the solid and liquid phases.

Embodiment VII

Table 20 shows the composition of an A357 material which is an aluminum alloy material. The aluminum alloy material was cut off from a high quality long continuous casting produced in a continuous casting process. In producing the casting, the spheroidizing treatment of a primary crystal α-Al was performed. The aluminum alloy material has a diameter of 50 mm and a length of 65 mm.

                  TABLE 20                                                         ______________________________________                                                   Chemical constituents (% by weight)                                  Al alloy material                                                                          Si      Mg     Fe    Sr   balance                                  ______________________________________                                                     7.9     0.57   0.1   0.034                                                                               Al                                       ______________________________________                                    

The aluminum alloy material was subjected to a differential scanning calorimetry (DSC) to provide a result shown in FIG. 72. In the differential thermal analysis thermograph d of FIG. 72, there are a first angled endothermic section e due to an eutectic melting, and a second angled endothermic section f due to a melting of a segment having a melting point higher than an eutectic point. The temperature T₁ at a rising start point g in the first angled endothermic section e is equal to 556° C., and the temperature T₇ at a dropping end point u is equal to 580° C. The temperature T₅ at a peak v in the second angled endothermic section f is equal to 598° C.

Then, the example A₁ was placed into the heating coil in the induction heating device and then heated under conditions of a frequency of 1 kHz and a maximum power output of 30 kW to prepare a semi-molten example A₁ having solid and liquid phases coexisting therein. In this case, the solid phase proportion was set in a range of 40% (inclusive) to 60% (inclusive).

Thereafter, the semi-molten example 5 was placed into the chamber 6 and passed through the gate 7 and charged into the cavity 4, while being pressurized under conditions of the casting temperature of 595° C., a moving velocity of the pressurizing plunger 9 of 0.2 m/sec and a mold temperature of 250° C. A pressurizing force is applied to the semi-molten aluminum alloy material 5 fill in the cavity 4 by retaining the pressurizing plunger 9 at the terminate end of the stroke, and the semi-molten aluminum alloy material 5 was solidified under the pressurization to provide an aluminum alloy casting A₁.

Five aluminum alloy castings A₂ to A₇ were also produced in the casting process under the same conditions as those described above, except that the casting temperature T₆ was varied.

Each of the aluminum alloy castings A₁ to A₇ was subjected to a T6 treatment which comprises heating for 5 hours at 540° C. followed by water cooling, and heating for 5 hours at 170° C. following by water cooling. A tensile test piece including a parallel portion having a diameter of 4 mm and a length of 20 mm and a JIS No. 3 Charpy impact test piece were fabricated from each of the aluminum alloy castings A₁ to A₇ and subjected to a tensile test and a Charpy impact test.

Table 21 shows the casting temperature T₆, the test results and the like for the aluminum alloy castings A₁ to A₇.

                                      TABLE 21                                     __________________________________________________________________________                       Presence 0.2%                                                    Casting                                                                              Cooling velocity                                                                       or absence                                                                          Tensile                                                                            proof                                                                              Breaking                                        Al alloy                                                                           temperature                                                                          during eutectic                                                                        of primary                                                                          strength                                                                           strength                                                                           extension                                                                           Charpy impact                              casting                                                                            T.sub.6 (°C.)                                                                 reaction (°C./sec)                                                              crystal Si                                                                          (MPa)                                                                              (MPa)                                                                              (%)  value (J/cm.sup.2)                         __________________________________________________________________________     A.sub.1                                                                            595   77      absence                                                                             325 265 12   6                                          A.sub.2                                                                            590   79      absence                                                                             320 260 12   6.2                                        A.sub.3                                                                            585   81      absence                                                                             319 254 11   5.8                                        A.sub.4                                                                            580   83      absence                                                                             315 250 9    5.1                                        A.sub.5                                                                            577   85      presence                                                                            342 311 7    3.9                                        A.sub.6                                                                            575   85      presence                                                                            345 320 6    3.5                                        A.sub.7                                                                            570   86      presence                                                                            350 325 5    3                                          __________________________________________________________________________

FIG. 73 shows an essential portion of a cooling curve for the aluminum alloy material in the course of production of the aluminum alloy casting A₃. The temperature at a start point y₁ in an eutectic reaction is 552° C.; the temperature at an end point y₂ is 535.8° C.; and an eutectic reaction time z between the start point y₁ and the end point y₂ is 0.2 seconds. Therefore, the reaction rate during the eutectic reaction is 81° C./sec.

FIG. 74 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₁. It can be seen from FIG. 74 that the metallographic structure of the aluminum alloy casting A₁ is formed of light-color primary crystals α-Al, and dark color eutectic-structure portions filled between the primary crystals α-Al, and includes no primary crystal Si (an alloy element) precipitated therein.

FIG. 75 is a photomicrograph showing the metallographic structure of the aluminum alloy casting A₅. It can be seen from FIG. 75 that a large number of black and angular primary crystals Si were precipitated in the eutectic structure and the primary crystal α-Al.

FIG. 76 is a graph illustrating the relationship between the casting temperature T₆, the cooling rate during the eutectic reaction, the breaking extension, and the Charpy impact value as taken from Table 21. In FIG. 76, points A₁ to A₇ correspond to the aluminum alloy castings A₁ to A₇, respectively.

For the aluminum alloy castings A₁ to A₇ shown in FIG. 76, if the casting temperature T₆ for the aluminum alloy material is set in a range of T₇ (580° C.)≦T₆ ≦T₅ (598° C.), the aluminum alloy material is brought into a semi-molten state and moreover, the solid phase proportion in the semi-molten aluminum alloy material is relatively lowered, i.e., the amount of the liquid phase is increased.

As a result, in the eutectic reaction of the semi-molten aluminum alloy material, the relatively large amount of liquid phase has a relatively large latent heat and hence, the cooling rate for the liquid phase is reduced. Thus, it is possible to avoid the precipitation of the primary crystal Si.

Consequently, each of the aluminum alloy castings A₁ to A₄ has a high ductility and a high toughness, as compared to the aluminum alloy castings A₅ to A₇ and thus with those produced with the casting temperature T₆ set at a level lower than T₇ and having a primary crystal Si precipitated therein.

The alloy material in each of the embodiments is not limited to the aluminum alloy material. 

What is claimed is:
 1. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature for said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, a peak value E₁ of a first angled endothermic section generated by eutectic melting of said eutectic segment is larger than a peak value E₂ of a second angled endothermic section generated by melting of said non-eutectic segment; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak value E₂ of said second angled endothermic section.
 2. A thixocasting process according to claim 1, whereinin said differential thermal analysis thermograph, a dropping line segment of said first angled endothermic section and a rising line segment of said second angled endothermic section are connected to each other in an area of endotherm higher than a basic line which interconnects a rising start point of said first angled endothermic section and a dropping end point of said second angled endothermic section.
 3. A thixocasting process according to claim 2, whereinin said differential thermal analysis thermograph, the gradient of said rising line segment of said second angled endothermic section is more moderate than the gradient of said dropping line segment of said first angled endothermic section.
 4. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature for said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, a ratio E₁ /E₂ of a peak value E₁ of a first angled endothermic section generated by eutectic melting of said eutectic segment to a peak value E₂ of a second angled endothermic section generated by melting of said non-eutectic segment is in a range of 1<E₁ /E₂ <2.5; a difference T₂ -T₁ between a temperature T₁ at a rising start point in said first angled endothermic section, and a temperature T₂ at a dropping end point in said second angled endothermic section, is in a range of 10° C.<T₂ -T₁ <120° C.; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak value E₂ of said second angled endothermic section.
 5. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature for said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, a relationship of E₁ >E₂ is established between a peak value E₁ of a first angled endothermic section generated by eutectic melting of said eutectic segment and a peak value E₂ of a second angled endothermic section generated by melting of said non-eutectic segment; said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak value E₂ of said second angled endothermic section; and when a temperature straight line interconnecting said peak value E₁ of said first angled endothermic section and a temperature graduation corresponding to said peak value E₁ on an axis of heating temperature intersects, at a first intersection, a basic line which interconnects a rising start point in said first angled endothermic section and a dropping end point in said second angled endothermic section, and when a dividing line bisecting the segment of said temperature straight line lying between said peak value E₁ of said first angled endothermic section and said first intersection intersects, at a second intersection, a rising line segment of said first angled endothermic section lying between said rising start point and said peak value E₁, a relationship of .increment.Tb/.increment.Ta≦0.68 is established between .increment.Ta (=T₄ -T₁) and .increment.Tb (=T₃ -T₁), wherein T₁ represents a temperature at said rising start point, T₃ represents a temperature at said second intersection, and T₄ represents a temperature at said peak value E₁ of said first angled endothermic section.
 6. A thixocasting process according to claim 5, whereina relationship of Sm/St≦0.365 is established between (1) an area St of a region surrounded by said basic line, a first temperature straight line corresponding to said temperature straight line, and said rising line segment, and (2) an area Sm of a region surrounded by said basic line, a second temperature straight line which interconnects said second intersection and a temperature graduation corresponding to said second intersection on said axis of heating temperature, and the portion of said rising line segment lying between said rising start point and said second intersection.
 7. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic segment to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, a relationship of E₁ >E₂ is established between a peak value E₁ of a first angled endothermic section generated by eutectic melting of said eutectic segment and a peak value E₂ of a second angled endothermic section generated by melting of said non-eutectic segment; a relationship of Sm/St≦0.365 is established between (1) an area St of a region surrounded by (i) a basic line which interconnects a rising start point of said first angled endothermic section and a dropping end point of said second angled endothermic section, (ii) a first temperature straight line which interconnects said peak value E₁ of said first angled endothermic section and a temperature graduation corresponding to said peak value E₁ on an axis of heating temperature, and (iii) a rising line segment of said first angled endothermic section lying between said rising start point and said peak value E₁, and (2) an area Sm of a region surrounded by (i) a second temperature straight line that interconnects (a) a second intersection of said rising line segment with a dividing line which bisects a segment of said first temperature straight line lying between a first intersection of said first temperature straight line with said basic line and said peak value E₁ of said first angled endothermic section and (b) a temperature graduation corresponding to said second intersection on said axis of heating temperature, (ii) a portion of said rising line segment lying between said rising start point of said first angled endothermic section and said second intersection, and (iii) said basic line; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak value E₂ of said second angled endothermic section.
 8. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature for said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, there exist a first angled endothermic section generated by a eutectic melting of said eutectic segment and a second angled endothermic section generated by melting of said non-eutectic segment; a difference T₅ -T₄ is in a range of 20° C.≦T₅ -T₄ ≦80° C., wherein T₄ represents a temperature at the peak of said first angled endothermic section and T₅ represents a temperature at the peak of said second angled endothermic section; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section.
 9. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature of said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, there exist a first angled endothermic section generated by eutectic melting of said eutectic segment and a second angled endothermic section generated by melting of said non-eutectic segment, said second angled endothermic section having a sloping portion between a peak of said second angled endothermic section and a dropping end point in said second angled endothermic section, said sloping portion extending along a straight line interconnecting a rising start point in said first angled endothermic section and said dropping end point in said second angled endothermic section; a proportion Rt (Td/Tc×100) of a temperature range Td between the peaks of said first and second angled endothermic sections and a solid-liquid phase-coexisting temperature range Tc between said rising start point in said first angled endothermic section and said dropping end point in said second angled endothermic section is in a range of Rt≧69%; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak of said second angled endothermic section.
 10. A thixocasting process comprising the steps of:heating an aluminum alloy material including a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature for said alloy material to a casting temperature T₆ to prepare a semi-molten material having solid and liquid phases co-existing therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, there exists a first angled endothermic section generated by melting of said eutectic segment and a second angled endothermic section generated by melting of said non-eutectic segment; said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at a peak of said second angled endothermic section; and a ratio α/β between the following two average temperature rising rates α and β is set in a range of α/β>1, wherein α is the average temperature rising rate for the alloy material between a rising start point and said dropping end point in said first angled endothermic section, and β is the average temperature rising rate for the alloy material between said dropping end point and said casting temperature T₆.
 11. A thixocasting process comprising the steps of:heating an aluminum alloy material which includes a eutectic segment and a non-eutectic segment having a melting point higher than the eutectic temperature of said alloy material to a casting temperature T₆ to prepare a semi-molten alloy material having solid and liquid phases coexisting therein; subjecting said semi-molten alloy material to a pressure molding to thereby fill in the cavity of a casting mold with said semi-molten alloy material; and solidifying said semi-molten alloy material under said pressure; and wherein said alloy material has a characteristic that in a differential thermal analysis thermograph, a peak value E₁ of a first angled endothermic section generated by eutectic melting of said eutectic segment, and a peak value E₂ of a second angled endothermic section generated by melting of said non-eutectic segment, are in a relationship of 1<E₁ /E₂ <2.5; and said casting temperature T₆ of said alloy material is set in a range of T₇ ≦T₆ ≦T₅, wherein T₇ represents a temperature at a dropping endpoint in said first angled endothermic section, and T₅ represents a temperature at said peak value E₂ of said second angled endothermic section.
 12. A thixocasting process according to claim 11, whereinin said differential thermal analysis thermograph, a dropping line segment of said first angled endothermic section and a rising line segment of said second angled endothermic section are connected to each other in an area of endotherm higher than a basic line which interconnects a rising start point of said first angled endothermic section and a dropping end point of said second angled endothermic section.
 13. A thixocasting process according to claim 12, whereinin said differential thermal analysis thermograph, the gradient of said rising line segment of said second angled endothermic section is more moderate than the gradient of said dropping line segment of said first angled endothermic section.
 14. A thixocasting process according to claim 1 or 11, whereinin said differential thermal analysis thermography, the gradient of a rising line segment of said second angled endothermic section is more moderate than the gradient of a dropping line segment of said first angled endothermic section. 